Below are the light values for my current (soon to be chopped) grow. I grow in ambient CO2. If you're adding CO2 to your environment, your plants should thrive on PPFD's at least 25% higher.
In the early stages of a grow, I can sample just the top of the plant but, as the plan matures, I'll take nine samples and, in flower, I sample each of the buds. For this grow, the PPFD values > day 60 are lowered because the "front row" of the plant was about half a foot lower than the colas in the middle and back of the plant.
In terms of under canopy lighting, I haven't looked into that much but I don't see a reason not to try it. In terms of precedence, given a setup like you have, I'd be putting at least 1200µmol on the canopy and then adding in the under canopy lighting.
When to increase light levels? Increase your light levels as soon as you want to improve the growth of your plant. The purpose of the vegetative state is to build the structure of the plant so that it will support the maximum number of flowers. By giving your plant a lot of light, you will tend to have shorter plants, limited internodal space, with lots of branches and a high number of small leaves.
Plant shape improves as light levels increase but you can only make a plant so "shapely".

On the other hand, in practical terms, there's no limit as to how much light you can give a cannabis plant when running CO2. I grow my photos at 1100± but when running enhanced CO2, researchers have given cannabis 2000µmol and have repeatedly demonstrated that yield increases in a linear/almost linear manner (depending on whose research you read) as light levels increase.
I would recommend a silica supplement because it tends to produce plants with stronger branches. Many of my colas end up being quite large (8-12") so the silica helps them support the additional weight of the large cola. The other benefit is that silica makes plants more resistant to mold.
This thread makes it very easy to DIY.
If your main light won't get you to 1200µmol+, I'm happy to recommend the Spider Farmer Glow R80. They're very inexpensive and add a fair amount of 660nm light. There's no reason to use them only in flower. With the exception of veg lights, all cannabis grow lights generate a lot of 660nm so you're just giving your plant the opportunity to make more food.
I added the R80's to my current grow and was able to lower the wattage on my Growcraft X3 from 300 watts to 160 watts even though the R80's draw only 80 watts. The savings comes, perhaps, from the R80's being newer diodes (that's a supposition on my part) and/or because red diodes are very electrically efficient. The drop in ambient temperature of 2° helps me keep the temps of the flower tops below 78°, a very important number when it comes to preserving secondary metabolites.
An excellent source of information is at the link below. I've spent at least hundreds of hours reading research and watching videos on cannabis cultivation and the presenter, a PhD candidate under Bruce Bugbee, does an excellent job providing a concise summary about getting the most out of a cannabis grow.
Forgot to ask — where did you get your hydro set up from? Is that from PA Hydro?