I'm not doing calculations. I'm dosing & testing using high quality professional lab equipment. I'm a professional with nearly 10 years of water treatment experience.
Well I've only been doing this since 1970; so, I defer to your experience. However in my laboratory we do it this way:
You need at least three measurements: the sample, a reference standard and a blank.
The sample is your aquarium water.
The blank is distilled water ( no nitrates present)
The reference standard is a solution that you make up with a known concentration of NO3-
You could take 10.12 g of KNO3 and dilute it to 82 gal. That would be a known concentration of 19.997 ppm NO3- but that would waste a lot of water.
I weigh about 408 mg of ACS grade KNO3 on my analytical balance then dilute it to 250 ml with distilled water. I take 10 ml of this solution and dilute it to 0.500 L with distilled water. The final concentration of NO3 in this solution is:
408 mg KNO3 *1/250ml * 10ml * 1/.5L *62.005 (g/mole NO3-) / 101.103 (g/mole KNO3) = 20.02 mg/L of NO3- = 20.02 ppm NO3-
The test I use requires 5 ml of sample; so I use three matched cuvettes. In the first I add 5ml of distilled water, the second has 5 ml of the reference and the last has 5 ml of the sample.
I conduct the reaction. After 10 minutes I set the spec wavelength to 560 nm and set 100% transmission level using the blank.
I measure the absorbance (A) of the sample and reference at 560 nm.
The concentration of the sample is:
A(sample)/A(reference) * 20.02 ppm
In order to make an accurate reference solution you need to know a lot of chemistry, or you could use the Fertilator!
I have never found the fertilator to make an inaccurate calculation but then, what do I know. I only get good results?