I have done some testing in this area due to the wide range of opinions about the topic. I asked people who I purchased plants from as it seemed to me that if they were good enough at growing plants to sell them, they must know something. I also tried various levels of FE over a couple of months.
My own results found that .5PPM is best for me. In my tank FE is the limiting factor for growth as I have many fast growing plants such as glosso, riccia, and hydrocotyles. I also found that if I overdose FE the only problem is that staghorn algae starts to appear. This is remedied by ceasing iron for a week and then dosing at a lower level.
Note: Results may vary depending on the type of plants you have.
hope this helps
30 Gal, Pressurized C02, dosing KNO3, FE, Trace, Light 4-6 WPG