That's a difficult question Tallslim. I don't what tpe of fertilizer you use but we use special fertilizer for marijuana plants from the speciality shops here in Holland. It's written on the package how much you need to give to your plants.
The easiest way to tell if your at your maximum nutrient level, is to look at the leaf tips. When the leaf TIPS start to turn brown, and 'burn', thats your maximum food level, back it off a lttle bit from their. It's trial and error, once you find out what the maximum is, always use that, for that strain.
correct- NPK Ratios>> start seeds at 10:10:10, then 3 weeks into it, go to 20:10:10, then once calyx develops switch to 10:20:10.
I personally use Fox Farms Grow Big for veg & Bat Guano for flowering.
My plants love it. Once in a while I treat them with a little Fulvic Acid which helps in good root development.