Buy a nutrient package and stick to the recomended feed scedules.Alot of preparation goes into finding how much to add at what week the plant is in be it flowering or veg.There is no point in thinking that phos will increase yeild of the overwhole crop.It is these kind of questions that to me are utter madness why don't you just get a feed a weel established one like sensi pro advanced nutreints vitalink is the one that i use and the plants always produce for me well the ammount that i need for my medical needs.It works out about 30 pound for 4 ltrs of vitalink you will need a mix of a and b veg same bloom this will last round about 2 grows with 12 plants on the go.Has long you aint one of the over watering growers there is no need to water more than once a week and about day 5/6 the plant will start to dry out let it it will be fine this will benefit you by the plant sending out new roots in search of water and feed[bigger the root mass bigger the yeild].