I do not think there is a residual etc or even a amount to give that really addresses how much. Test kits are very poor in the area of trace metals like Fe.
The closest way to do this is to add say 5mls per 75 liters 3-4x week, that's the max amount you will need.
Slowly lower from there, say 3x a week, then perhaps 2x a week with lower light.
Some divide it up into daily dosing.
So say you have a 75 liter tank, adding a total of 15mls per week.
15/7= about 2-3 mls daily.
If you prefer less frequency: 3x a week x 5mls.
What you do not want to do in a higher growth CO2/Excel enriched tank: add 15mls after the water for the entire week.
That's okay, actually about 5mls for a non CO2 tank, but not CO2 planted tanks.
The test kit issue goes like this: does not test for the bioavailable fraction, it drops out of solution very rapidly(Say seachem flourish, TMG is better) within hours.
So it does not last long and is certainly not stable over a one day time frame. ETDA chelators are even worse.
So the argument becomes how much we add and have in solution at any one time is not important. what is important are the plant's health, thus there is little/no correlation between plant health and residual Fe levels.
Note, we do not test for all the other traces either, we merely use Fe as proxy for all the other trace metals etc, so we may as well use the plant health method.