Well, you can measure the effectiveness, but it's not something the aquarist might do unless they are really into it.
If you are honestly into it, PM, and I can tell you how it'll likely cost you a few hundred $ for the equipment, but it's far from impossible.
Still, the point here is really why not use CO2 gas?
It works very well even at very low ligth(you get more growth using CO2 than Excel at ANY light level).
Excel is simpler to use, I think that is the best solution for some and the trade off they like. Many are very intimidated by CO2 gas, and try to avoid using it like the Plague.
Learn how to use it and learn it well.
That is more important and will give you a lot more gains in the hobby than any other nutrient.
Excel and CO2 are both safe when used correcty, but many simply do not give either the respect they both deserve and kill their fish due to outright carelessness. Neither of these things directly kill fish or harm them when used correctly, aquarists do that
I've yet to have met a single person that's killed their fish/shrimp with KNO3, KH2PO4, but many will belly ache endlessly about how bad, hard, how algae is induced etc by NO3, PO4, or anohter myth about K+ stunts their plants, then brush off CO2 like it's nothing.
Excel is not a bad thing to use here and there to get the CO2 dialed in or for smaller tanks ..........as mentioned already.
Nor is it a bad solution for say a 20 gal lower light tank all by itself.
Slower growth is the goal or many, so low light + Excel works well, and you can do a semi low tech tank and also still do large frewquent water changes if you wish, for a pure on CO2/no carbon enrichment, most do not do water changes except perhaps once every few months.
So there's the trade offs to really consider. There's no cut off point really, some use it with CO2 at high light, but using Excel alone at high light is pointless, you are wasting the light energy, if you want faster growth(which is why you have higher light right????!!!), then add CO2 gas, not more light.
All more light means is that you have more CO2 and nutrient demand and faster rates of growth. Most folks honestly do not want that.
You do not get better colors, better growth, you just get faster growth at higher light. More is not better in other words.
CO2 is the key player here, so work on that.
Lower light-> buy a gas CO2 system first. Then see. If the growht rates are too slow, add ADA AS+ good water column dosing, frequent water changes.
If that is still too slow, then, lastly................consider more light.
An aquarist is likely very impatient if this is the case and will make many mistakes so I do not have high hopes if they go that route.
Most simple assume more light is better for planted tanks, and do not consider CO2 as important, when in fact, it's much more in general.