I use Seachem Iron in a 55 gallon and it works great. Easiest way to tell how much to dose is paying close attention to your plants. I started dosing iron every three days once I noticed new leaves where pale on several plants. I increased the frequency a little each week until I saw improvement and found a rough sweet spot.
Since requirements vary depending on several factors (plants, lighting, water source) it's much easier to be patient, learn what to look for, and slowly increase dosage. I now dose seachem iron three times a week and consider my tank light-moderately planted.