dump and guess
I have gone to dry dosing, since I always got a nasty slimey stuff growing in the solution, even after I started using muratic acid as an additive.
I use the tiny spoon that comes with the Red Sea iron testing kit, I have no idea how much it is. I just watch the red melon sword, if it is too little, the new leaves are shorter, if it is,,, what? enough? too much? the leaves are really big and threaten to outgrow the tank. I don't want it too big, nor too little... guess I'm looking for the Goldilocks solution.
I just add it to a cup of tank water and dump it in. Once or twice a week, maybe 3x if I think of it.