I don't use a calculator. I have worked out, through testing before and after addition of supplements, how much of a supplement will raise the concentration. Its actually easier than the calculator.
If 1 cap full of product A raises calcium by 25 ppm, then 2 cap fulls will raise it 50 ppm. See how easy that is?
The problem is that if you don't add it properly you don't get an increase of 25 ppm, you get a handful of sand in your sump, or worse, a cap full of scale build up in your pump.
Gary
125 SPS, 75 gal. LPS/softie reef, 9 gal. Nano