Well, CO2 is the lion's share of the fertilization we use, 40-45% of the plant's dry mass comes from the CO2.

N only 1.5%, K+, about 1% and so on......

You might try the Baking soda Drop checker method, your scale is plenty accurate enough.

You can always make 10liters instead of 1 liter an gain an extra significant digit.
You can toss the remaining 9 liters down the drain if you want etc.
Some what wasteful but will give you more accuracy.

Concerning the net and other information: I want to say that I may preference everything I say as tenative. But......in light of a better alternative, I stick with the most probable solution/reason. Step wise incremental improvements to hypothesis and reasoning help a great deal to arriver at a better answer, even if it's not the right one, which often happens in light of new evidence. Much of the older information was as right as they could get it at that particular time. My only wonderment was why did they not test their hypothesis they developed?
Seemed odd to me.

You need to be able to control the CO2, since controlling the nutrients is easy with EI, light control is also easy(timer etc). If you cannot control CO2/nutrients etc, you cannot test either.

So main thing for the algae you have, clean it off and fluff it off as much as you can, get 2 otto cats, and do a larger than 50% water change, say 15Gal or more each water change, or 2 water changes a week till the algae is under control.

CO2 is all that is left after that.

Regards,
Tom Barr