Using Bru'N water, my water has a base Net Mash Acidity (NMA) of 22.4 for a random grist that is in the spreadsheet.
When I add 1.0 g/gal CaSO4 I get 61.4 ppm Ca, 147.5 ppm SO4, and the NMA is 48.2
When I add 1.44 g/gal MgSO4 I get 37.5 ppm Mg, 148.3 ppm SO4, and the NMA is 35.4
So I've added the same amount of SO4, but got half the increase in NMA.
Maybe Martin can explain why that is. It could be because of what Wild Knight is talking about above, I don't know.