This info from Beer and Wine Journal IPA Experiment may be helpful...http://beerandwinejournal.com/ipa-experiment/
"Measuring the tiny amounts of gypsum required could be a problem. One way around this would be to make a known gypsum solution and add this. However, the solubility of gypsum is around 2.4 g/L in pure water. As such, you’d need add a total of just less than 2.0 qts. (2.0 L) of this solution to the beer by the end of bottling. So, when adding gypsum, just do your best to measure out the small amount. Then, wet it into a slurry. This will still likely raise some foam (and lower your level of dissolved CO2) when you stir it in, but it won’t dilute the beer as much. Incidentally, the solubility of calcium sulfate increases in colder water, contrary to how most solids behave, so don’t try to use hot water to make the slurry. The solubility of gypsum is increased in acidic solutions, so it’s possible that you could make an acidified known solution of gypsum, but then you’d also be dosing the beer with whatever acid you used."