IMHO, I wouldn't add any N to your FFOF soil yet......it's loaded with what your plant needs for the first couple of weeks at least. I 've been useing it for years and I never add anything until I go 12/12, unless I veg for 5-6 weeks and they show they need something..... FF is pretty hot soil already(why it's $18 a bag right now..).
growinman