Bill, I've tried testing iron for many years with FW systems and have a better solution than testing.
Using a set volume to tank ratio is much better.
Say 3mls per 20 gallon of tank 2x a week etc.
I really would strongly argue against any type of Fe test kit that you can relate consistent results to consistent growth in any plant or algae.
I have used Lamott and Hach kits that run about 65-90$. Other kits are pretty much worthless.
The lab has autoanalyzers but iron is funny and requires other methods.
Basically, the test kits do not tell you what it is you need to know.
Fe is very funny, it forms some 12-20 inorganic comlexes in water, I will say there are more organic complexes and these vary in their bioavailabilty which is the key question here. Totla Fe is not the same as bioavialable.
You advice is good about starting low and slowly adding more till you get a negative response.
This is what most suggest but rather than testing with kits, I would suggest trying the tank volume, mls per unit time as a better method.
I have had trouble with adding KNO3 + KH2PO4 + traces all at one time or within 2-3 days of eachother but if I add just KNO3 and traces etc things are fine or if I add just KH2PO4 and traces things seem fine also.
I cannot add all three without problems.
Hope this helps,
Reghards,
Tom Barr