I used iron filings myself.

I think it's hard to determine unless you have a Redox probe and/or a DO probe.
Then you can measure the amount of reduction that is optimal for Fe reduction.

About 200mv range or so.

But...........what about when the Fe is in aerobic water column? Then it gets oxidized back to rust again.

So the leaves never get it unless................they transport a generally non mobilie nutrient all the way from the roots to the apical tip. Or you use a liquid source like DTPH, gluconate, Glycine, ETDA etc.

I use the shelf life in the water column based on KH for each chelator type rather than an energetics model. I think simply having the Fe in any form is best, but you can also address the energetics by comparign a weak and strong chelators, that would provide low energy, while the strong chelator would provide longer exposure time/frequency.

The best benefits of each theory.
That's what the Barr's Matrix will have that Greg sells, at Aquarium Plant Food - hobbyist taking care of hobbyist … | Greg Watson's Planted Aquarium Fertilizer

Hopefully this June.

Regards,
Tom Barr