I think what is lost on a lot of people here is that the Alchemist is targeting a final beer hardness of 750mg/L,
NOT the starting brewing liquor.
Malt adds significant amounts of Ca & Mg to the beer, so the final calculated hardness ends up being 750ppm. This has been tested by Ward Labs:
https://www.homebrewtalk.com/f14/heady-topper-results-ward-labs-481031/
If you are trying to achieve 750ppm hardness via calculation by ONLY by the addition of calcium salts to your strike/sparge water and/or kettle, you are vastly over-mineralizing your water. Remember that the influence of Mg on hardness calculations (as CaCO3) has a much bigger influence than Ca due to their respective atomic weights. This means that salt additions at the Alchemist are in reality considerably more moderate than what is being proposed here, and are more in line with accepted brewing practice.
The way to find out the closest approximation to what the Alchemist is actually doing would be to brew the clone in de-ionized water and have the final beer analyzed by Ward Labs. The CaSO4 and/or CaCl2 additions could then be calculated by difference based on the results obtained for the real thing.
Bottom line is that some homebrewers appear to be adding way too much gypsum, at least if your goal is to truly clone this beer.