You can measure the presence of a mineral in water, but you cannot measure the amount stored in tissue or complexed in substrate, unless you work with tracers and destructive tissue analysis. Hence the best instrument by far is the human eye.
Managing
plant food can be a challenge in high energy tanks. That's the appeal of EI dosing; ensuring unlimited supply of all minerals, without much regard for ratios and with the belief that plant food by itself is not responsible for algae. I second the latter.
The red plants you are referring to are no doubt perennial stem plants such as macrandra, wallichii, inclinata, rotundifolia, etc. In low CO2 conditions, these are all prone to shedding leaves from bottom to top. Hence the single most effective way to delay this is through CO2 injection.
The most likely reason for algae in these tanks is not so much the minerals/plant food (algae hardly need any to grow anyhow), but rather the combination of high light intensity and organics produced by plants in overdrive.
The red pigment is caused by high light intensity (or some other stressor such a nitrogen depletion). Hence, you don't want to sacrifice on high light. That leaves constant dilution of organics through
water changes.
A very nice way to start up a high energy tank is to work with CO2 injection under low to moderate light intensity. This gives a much slower but dense growth, without the red colors ...and without the algae. Once your tank is highly dense, crank up the light intensity, adopt a more intensive water change regime and turn green to red.
I've kept large high energy tanks in the past and gave up on them 5 or so years ago, becoming tired of the trimming, re-planting and the water changes. Recently, I started a high energy tank myself. Only 10 gallons.
Cheers.
