May I make a suggestion? On just a couple, hand water your same mix. But put it to 6.3 6.4 for a couple waterings. See if the pH let's the calcium and mag uptake better. Just a suggestion. I am going to try the same thing. However I am in Coco and normally run 5.9-6.1. But Remo nutes suggests 6.3 in Coco. Promix needs higher pH than Coco. So I would try 6.4 on a couple plants. The only way to know is to try different things. Obviously pH is extremely important, So for some plant under cobs I will be trying 6.3pH on a few plants just to see if they respond differently. Cobs are more intense than they look. That is the main issue, I think I might have too much light for the taller plants. However, the buds form way faster closer.my tallest 3BOG really is bulking up dailynnow. And it is by far the tallest. My next round I will be starting my plants in the cob tent, so they do not get to tall and the lights can be further away. Actually I am going to move the 2 tall ones to a larger tent, but still use cobs..just more space. My 4x4 is crazy bright, too bright for the tall ones.now I will have more room for the smaller ones.
of course your going to want to learn or at least understand the CEC. the cation exchange capacity ,which in many cases calcium runs thicker through some of these things than others. but we all know magnesium crystals or Epsom Salts which rarely if ever effects soil PH is used in simple moderation. but if you were to combine this with ,say, Sul-po-mag,bone meal , an organic source of gypsum ideally ,sweet lime and pellet lime.oats,rice,cereal brown rice and soy flour.also soybean meal is also decent but cottonseed is also ok. but releases far far slower than soy meal. also, look at legumes too. alfalfa...so alfalfa meal as well also contains these things and they in turn with these other materials comprises essentially 1/3 of a healthy well thought out totally amendable and sustainable soil, so works for me and I have had some success with amending my own soil with some of these things under a mars and two doses of cal mag at normal rates. not bad eh?! so in the same mix I would say each component should hold enough at roughly a cup(1.5 - 2 cups for some things) per item with a 20-25 gallon mix.. or a 30 gallon mix go to a 1.5 average. but the amounts and effectiveness will also boil down to composition and temp and unbroken and broken down materials and life availability as well. but overall it'll be effective. some more than others due to forementioned things 
