Nilo,
It's not terribly complicated at all. We make it complicated by our assumptions. On any given day your water supply may have any amount of nitrate and phosphate low or high. The problem is we have no control of what's in that supply. If we dose according to the EI schedule then we are sure that we have at least a certain amount of N P and K, regardless of what the water supply has. If you
don't dose then you essentially have a lottery. What happens when the plants use up the nitrate over the course of the week between water changes? As soon as the nitrate hits some minimum level plant growth becomes limited. The same with phosphate. If you dose the maximum amount for the amount of light and CO2 you have then you know that you will never run low.
A highly lit 50 USGallon tank should receive the following dosage (everything scales linearly - a 25 G gets half and a 100G gets double):
3X per week
==========
1/2 teaspoon KNO3
1/8 teaspoon KH2PO4
This dosing, combined with your ferropol dosing as discussed previously should ensure that you have slightly more than enough of the macro (NPK) and micro (Fe and other trace). If you are using tap water and if your tap is hard then you need not add Mg or Ca. The reason we can get away with Mg and Ca from the tap is that the quantity of Mg and Ca plants use is miniscule compared to the amounts of NPK and Carbon that they use. As an analogy, think of NPK as Meat, CO2 as bread/potato and ferropol as Vitamin tablets. With this analogy you can imagine the amount of meat you require compared to the amout of vitamin tablets you would consume.
If, as you stated, you wish to maximize growth rates then you can only accomplish this by maximized feeding. Ignore the level of nitrate and phosphate in the water supply and dose as if they weren't there at all. Save yourself some dough and eschew the Brighty for the powders you can find here:
http://www.aquaessentials.co.uk/index.p ... th=145_146
Having said all that, I have to admit that CO2 is a bit complicated. It is simplified somewhat by use of a drop checker with 4dkH water. Are you using this? If CO2 is poorly applied the other nutrients will have limited effects (think of the analogy above).
Cheers,