The answer to the basic question of "can I tell how much of my solutions total EC/ppm are macro-nutes" turns out to be, Yes you can. There are hydro solution test kits.
I know this technical stuff gets pretty long so not sure many will stick to the end, especially if LIT, but some good info for those who do.
I put out about 10 calls to nute OEM's and professional growers to pose this question in the hope that I would get one to speak with. Ended up with a better response than anticipated, and had great dialog with several leading to the equipment manufacturer I seek, I'll link below. Not only can we measure it, they do.
I am new, but always thought it odd we place so much emphasis on these magical EC/ppm numbers when they don't really say anything about N,P or K amounts in a given solution. Forget the micro nutes for now, but come on..we should all know the basic macro's. To the plants, their N needs are as basic as our need for each breath we take to be 78% N. You take 2-3 breaths from us without it and our leaves are already yellowing, take 5-6 breaths and we are turning shades of blue, take 10-15 and we're dead. Thank goodness their metabolism is much slowed relative to us, or I'd be even a bigger plant murderer than I am.
Don't get to tied up in the technical definition of EC or TDS ppm, after all you only really need to understand a few things practically speaking.
First to know is that your reading is a snapshot of a solutions state for that moment. After all, if the system is working correctly and maintaining living plants, it should change measured levels with plants usage and should only be applicable for maybe 8-10 growing hours, give or take. These changes are also what facilitate hydro solution Ph changes which we all do say to measure daily.
Next to know is that if you know your EC, you pretty much know your TDS ppm. TDS is basically a math function of the EC. Even if your meter only displays TDS, its still probably reading EC and performing this math as the output. You can simply turn your meter upside down and see that there is no metal tube actually taking fluids into the instrument for gasification and analysis, not that you could fit all that into a small hand held like these anyway. NASA has some of the smallest of this type full analysis equipment, and its still the size of a shoebox using power sources that you or I would end up as a government guest at Guantamino Bay for possessing. An EC meter needs only a few things being, a regulated voltage source, a resistance probe and a thermometer. As you pointed out, some use the .500 number while others the .700 for math conversions, and then both grade the result against a temperature curve for the final display. This can be an important selection as nute manufacturing companies will state somewhere in their nute tables which they do use. If you are using Advanced Nutrients for example, they use .700. You need to select this to match on your meter to get the same levels stated in their nute charts/tables for any given combination of additives. I like using .500 better as a stoner just because even I can mentally divide by 2 ie..1000EC=500ppm etc. That result just isn't completely accurate because I can't mentally adjust to solutions actual temperature curve, but close enough. Here are some quick actual conversions I'll measure at 79* solution temp using the 500 number and you can see the temperature part doesn't change the quick divide by 2 very much; 2251EC=1123ppm; 1739EC=870ppm; 970EC=485ppm and 634EC=320ppm. I don't know about all meters obviously, but my Combo is very easy to go to program mode to not only select the 500 or 700 number, but you can even play with the Coefficient Beta or temperature curve if you want to.
Inherent in these two basic truths above about EC/ppm, is that I can give you a 1000EC which contains no N, P or K. We know its not true, but just assume for this example that all 3 of these elements had the same conductivity. I could mix twice the amount of K, with zero N and you would still read your 1000EC or 500ppm. There is an infinite number of combinations you could play in this hide the shell game, only a few would actually feed your plant while many would just kill it outright.
In summery, you would be hard pressed to show me why I should not specifically measure for at least N,P & K being new to growing, just like I do for Ph. A major difference becomes evident when looking at how the experienced growers manage this function relative to the newbies like me. They use that accumulated experience to be proactive based on site, sounds, weights and smells etc. Most of them are like asking a Chef how much salt to use in this recipe only to get a replay like 'a pinch' or 'just a tad' etc. I am generally reactive and trying to figure out two days late why my plant is turning this color or that and shriveling up, being I lack that very experience. I just don't see why for example, if you are on a two week solution change out schedule, and at the end of the 1st week I test for macro nutes and find one to be low relative to what was measured on initial mix, I would starve the plant of that for another week to maintain the schedule. I say do the plant a favor and give it what it wants while also giving yourself a greater yield. If you use a whole new mix every week, it seems equally valuable information to confirm nute levels at least until you gain confidence in the OEM's nute system you have chosen to use on that current grow.
Know for a fact that there can be a great difference between these OEM's when it comes to their formulas and what you get when mixing per their charts/tables. The US based as well as the larger internationals seemed a little more even keeled, but they are intermixed by these smaller guys who said stuff like "they did a full battery of tests in '88 and have not changed their formulas". Hell, I farted in '88 and it contained methane. If you tested a recent sample it would still exhibit methane I imagine, but hardly a certifiable result as to quantities or compositions. I won't be pouring this stuff into my bin.
This fact does lead to another important point for the new guys, and maybe even the experienced too. Take 30 seconds when mixing your next fresh batch up to secure EC & Ph readings on your base carrier solution before any additives are introduced at all. Many of these nute OEM's and pro growers are basing their formulas on whats available locally to them and may in no way pertain to you and yours. My city for example is on stored tank water in the winter, and goes to well water in the summer. I buy distilled from different stores and have had substantial differences, RO filters clog etc. etc. Just get a handle on what you are starting with in your locale.
The only downside I can find to measuring the actual nute levels in a solution is the initial equipment cost of around $285. This is really not that out of line when compared to what we spend on RO systems, lights, meters, grow boxes or tray systems etc., when considering that it measures the macro nutes essential to the life of a plant while a EC/ppm does not. The kit is also like buying your EC/ppm meter in the sense that the initial purchase includes the labware for the tests, and to maintain it you only then need buy calibration solutions, batteries and occasional probe replacements. My 4.0 & 7.0 Ph buffers for a 2 point cal, along with the OEM recommended 1418 m/s conductivity cal buffers are had for a fraction of the initial meter cost. This kit is the same as it comes initially with the labware and enough solutions to perform 50 Ph, 50 N, 50 P and 50 K tests. To restock these solutions for another round of 50 tests each is $93.
If you check down at the bottom of their linked page they also have an expanded kit which will test not only macro but micro nutes as well, for greater cost of course. They even have one you chop up plant matter to determine actual palnt usage. I am not so interested in micro nutes as macro, and if I chop up plant material it is going to be packed into a bowl and shortly introduced to fire with me as the vacuum pump so won't be getting them. It may take me until the end of the summer or so to save for this basic kit, but I intend to get it and solve some mysteries for myself. Check them out at:
Basic Kit;
Hydroponics 4-in-1 Test Kit
Advanced Kit;
Hydroponics Combination Outfit
Tissue Test Kit;
Plant Tissue Test Kits