Well, the equation for calculating total hardness is [(Ca/20)+(Mg/12.5)]*50

Assuming you are going to leave Mg alone and solving the equation for Ca tells you total Ca would need to be 261.6 ppm, so you need to add 186.6 ppm of Ca.

1 gram of gypsum in one gallon of water adds 61.5 ppm of Ca (as well as 147.4 ppm of sulfate).

Divide 186.6 by 61.5, and the result is 3.03. So you need to add 3.03 grams of gypsum per gallon of brewing water to bring your total hardness to 750 ppm.

Your resulting water profile would be:

261 ppm calcium

24 ppm magnesium

88 ppm sodium

647 ppm sulfate

92 ppm chloride

I've never brewed with a water profile that comes anywhere close to that, but it doesn't look good to me. Your mash pH will likely be far lower than desirable.

You could also use magnesium sulfate to get there, but at 24 ppm, your magnesium level is already getting pretty high. One gram of magnesium sulfate in one gallon of water adds 26 ppm magnesium and 103 ppm sulfate.

Personally, I would ditch the goal of 750 ppm total hardness, and I would either cut that water heavily (like 75%) with distilled water and add back a bit of gypsum (maybe like half a gram per gallon) or start with all distilled or RO water and build from there.

If you don't have one already, consider downloading a copy of Bru'n Water and read the "water knowledge" tab.