Check out this post on another forum. It is 7 years old now and the situation may have changed over time but when I was looking for an iron test kit other research I did at the time supported this person's claims... I would welcome any corrections to this information from those of us here that are more knowledgeable.
Heck, Shawn's dead.
Nice guy though.
He had little clue about about testing however.
Fe is one of the hardest.
Bottom line, do not even bother.
Most test based on the plants, give any routine about 3-4 weeks time to gauge whether you should add more/less etc.
Adding excess amounts has never shown any ill effects in any test I've done with traces.
I've never been able., nor have others, to correlate plant health and Fe dosing residual levels in the water column.
The issue is that it is gone before you have much of a chance to measure it.
What goes to the plant or what is complexed or preciptated is a huge question.
No one really knows without labeling the Fe using a stable of radioisotope and then measure how much was added vs how much % got into plant tissue(tissue analysis).
Not something hobbyist really do.
The test it extremely time dependent that we use and the levels of Fe do not correlate to plant health.
So do not waste your time.
Generally, a non limiting level is about 5mls of TMG 3-5 x a week a to an 80 liter tank with high light and CO2.
You might get away with less.
Flourish seems to have the same effects but I like the TMG a bit more.
You can alternate back and forth between them also.