How much do you feed them extra-PK with Biotabs ?

For later stages of flowering P,k is needed more. Not so much in the pre flower stage. I have hydroponics powder and a pk booster and they recommend, that you apply the pk booster after 2-3 weeks of flowering, so after pre flower ends. Same principle. You'll see in your plant when certain things are needed. But you can slowly start to apply your pk booster in small doses if you'd like. But, really its not needed until you reach fattening. Good Growing pal