I beleive the most scientific about their tanks use this Fe tester https://hannacan.com/hi746-iron-low-...eckerr-hc.html
I don't have a dutch tank so I never saw the need to get one but its as good as you get in our hobby. Range is up to 1ppm Fe which should be good for most tanks.
That's what I have. I have the one that has a 0-5PPM range and shows out to two decimal points. It's actually not so easy to use, but yes it is a nice tester.
I dose .1ppm iron daily (.7ppm weekly), but that is all gluconated (Seachem Iron). Because of my level one UVS, I cannot use chelated iron, such as EDTA or DTPA. Generally, you need to dose more gluconated iron that EDTA or DTPA forms because it is consumed by plants so rapidly. After going back and forth with Seachem on how to determine how much to dose (their directions are a little misleading), they advised to dose enough so that ferrous iron tests show zero iron after 20-30 minutes. During that time, the plants will uptake virtually all of it. In my case I could actually dose higher than I do and remain within these test parameters. This is because it is in the ferrous form and not the ferric form found in chelated versions, which are designed to last much longer in the water column (up to several days). Additionally, the gluc iron is not dependent upon pH levels.
That answers a lot of questions. It sounds like during the titration period, dosing should be during the day with lights on so that plants can uptake the iron immediately and can be retested 20-30 min later. I have been dosing an hour or so before lights on (which is convenient for me). I can see why they recommend daily dosing of it. Actually it seems critical in a high tech fast growing tank.
Since iron can't be transported inside the plant and it's gone in 20-30min, it needs to be re-introduced everyday for new growth to get enough iron.