Most stable pH down chemical ?

[baxbax]

Well calcium isn't immobile. It's semi mobile and issues will show up on whichever leaves are (were) growing fastest. But I'm sure we can confuse ourselves for years to come trying to chase deficiencies. Iron deficiency is my favorite one because it's pretty obvious and actually fixable. The rest usually involve a combination of factors.
But I find that it generally always boils down to this-

Make sure your ph going in is around 5.6-5.8.
Don't overfeeding and don't underfeed. Underfeeding can cause deficiencies as well, obviously.
Make sure there's runoff at least some of the time.

If you really want to get to the bottom of it and you're growing several plants then it's worth doing a side by side with a couple of them- different amounts of calmag for the two plants.
But in my experience it's not that easy to give them too much calmag. Your more likely to Od them with all the nitrogen in it before causing other issues.
And the times I've grown in coco and tried skipping the calmag I got rust spots like that.

If calcium is semi mobile i have peace of mind it is calcium deficiency I thought calcium is immobile as I searched in google , and yes with my 5.0 and 4.5 feeding ph plus reducing my calmag feed no wonder if I have cal mag def . trying to feed some cal mg

 

[Weaselcracker]

Just keep the ph steady at 5.6-5.8. Did we mention this before...
Ph is almost always the cause of 'mystery' problems.