I use 1.25-1.5 ml per litre of each a+b for small cuttings/seedlings, and up to 2.0-2.25ml per litre for bigger ones, seems to work just fine, haven't used pk13/14 alot but I normally use it at 0.3ml per litre starting at week 2 of flower or when stretch has stopped, up until flush seems to work...