Re: Iron - When is there enough
You are correct; because Flourish Iron contains iron in the plant- preferred oxidation state, which is ferrous iron (Fe2+), plants utilize it immediately, especially in high-growth situations.
The first thing that I would suggest is to test the reference solution that is included in your iron test kit. Doing so will rule out a test kit error, and this will also ensure that you are not overdoing it on the iron. I would hate for you to continue dosing and dosing in order to see a reading, just to find out that your kit was not functioning properly. I highly doubt that this is the case, but it will ensure that you do not induce iron toxicity with continued dosing.
If indeed the kit is giving an accurate reading, then you can increase the amount and/or frequency with which you are dosing Flourish Iron. Since some of your plants seem to be exhibiting signs of iron deficiency, then slowly increasing the amount of iron that you dose is completely safe, as it will not linger in the water column.
Beyond that, your dosing regimen looks great. As far as phosphates are concerned, there is obviously a fine line between enough and too much. It's best to try and maintain a phosphorus level of 0.05mg/L in a planted tank, which equates to 0.15mg/L of phosphates. As long as the algae is under control, then you can remove PhosGuard from the filter, so that there is some available for your plants.
Have a nice afternoon.