Personally, I don't see much point in that test.
I don't even use the phosphate test all that much, but it can be helpful as there are non-fertilizer sources of phosphate that can be quite significant, and plant uptake is somewhat substantial. In particular fish food/waste, and phosphate buffers (if used by your water supply) can add phosphate to your tank.
There's only a very tiny amount of iron in fish food, so most of the iron in your tank is going to be from fertilizers. Thus, you should be able to predict water concentrations fairly easily using the petalphile calculators.
The only thing the iron test would help you with is quantifying plant uptake. However, iron is a micronutrient, so it is used by plants in comparatively small quantities compared to N,P,K, and even S, Mg and Ca. It may be the most used micronutrient, but it is still a micronutrient.
This analysis is for farm crops, but the charts should give you a general feel for plant nutrient use, based on what they end up containing in their leaves...
Note that 100ppm is 0.01%, so you can use that when comparing the numbers in the micronutrients vs macros and secondaries.
Perhaps if you are running a specialized tank where iron levels are critical and demand is high, this might be worthwhile. However, for a common run-of-the-mill tank this is probably getting a bit more detailed than you need to.