Happens shortly after flipping to flower. Switched soil to fox farms 50/50 ffhf/of and ever since I’ve been dealing with this early flower to finish.
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When flipping what is your pH ppm run off? Just wondering trying to help troubleshoot the problem.
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6.3-6.5
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Id lean more towards 6.5 to 6.8 what are you amending with ?
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4-4-4 & 2-8-4 Gia Green
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Have you tried any CalMag? Just a thought
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Ive been using cal/mag yes.
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Looks like P deficiency to me. I also concur with @DanB pH is on the lower end. Once they start flowering its an increase in internal activities. I usually increase pH by .2 or .3 during flowering.
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Are you sure it’s not a zinc issue.
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Resembles a phosphorus deficiency and often accompanied by a calcium deficiency. Most Base nutrients have enough phosphorus to carry the plant through veg and the flowering period, deficiencies are typically root zone related as in the PH is out of range and preventing uptake. Calling in an export in this department for another opinion @Budbrother can you offer some words of wisdom 
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Zinc tends to have a more uniform effect to a whole leaf. The splotchy nature makes me think phosphorus. Just spitballing though. Can never be sure without more details.
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Doesn’t too much calcium create calcium phosphate? Which can’t be absorbed by the plant? Did you mean magnesium?
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An over dose of calcium can prevent potassium and magnesium uptake. I was stating that most often a phosphorus deficiency is typically accompanied by a calcium deficiency. What’s your dry amendment schedule and have you done a soil slurry test to check the mediums PH around the root zone?
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The last two pictures on first post show -Mo with the black spots. This is often brought on by cooler temperatures, So have you had any temp changes?
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This could be the advanced -Mo stage. Either that or there is an imbalance of P Ca. Hard to judge without knowing your regiment. I saw Gia amends being used plus calmag. Did you add gypsum or another Ca source?
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I’ve been using a mix of the same FF soil as you, both new and re-amended, along with Gaia Green’s dry amendments and some other little goodies along the way. I’m not good at evaluating problem signs but have had some similar to what I’m seeing in your pics minus the real dark spots. In my situation, I think it was mostly a pH issue since it’s pretty much gone away after dialing in the pH.
Another note - a local grow buddy of mine has been trying to get me to switch base soil away from the Fox Farms, he says it’s not compatible with Gaia Green fertilizers…but doesn’t explain. I’ve read a number of postings in growers threads that FF soil can be inconsistent, erratic (not the right word, but my mind isn’t awake at the moment 
). II’ve had relatively good results from it after working out my pH problems. 
Question if I may ask - what is Mo?
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molybdenum=MO
What was causing your PH issues?
High or low?
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