I'd stay away from
calmag at this point in the grow. The plants' calcium demand declines after bud set, and most calmags include more nitrogen than I want to add in late flower. When I use a PK booster, it's between weeks four and six, inclusive. That's for an eight week strain...I'll stretch that out if I'm running longer. I give mag sulfur through the week before chop. Starting at about 120 ppm in week four of flower, and ramping up to about 200 ppm in the penultimate week. Not saying that's right...just what's worked for me.