I believe the pp is CaSO4.
I also know it as gypsum, I'm more inclined to get it via that way, but if it's 100% CaSO4, or close, then use it, especially if very fine ground.
It does dissolve but takes more time than say CaCl2, but it's a lot more soluble than CaCO3.
Yes, that amount should relieve most any issue you have with GH, dose that amount 2x a week.
Since this thread is about using the PMDD liquid dosing method, generally daily, add 2 teaspoons of CaSO4+ 1 tsp of MgSO4 per week's worth of dosing.
Note, since it's less soluble, as is K2SO4, plus you have a larger tank.......you may consider using a larger volume, say 2-5 liters at a time for a 2-4 week batch for the macro's, and 500mls for the traces.
The confusing thing and at the same time the flexibility here is that you can customize the solutions to meet any goal or demands you desire for the tank.
This is one reason I went to EI type of dosing suggestions for folks, simple made it easier for folks.
There are trade offs, but not that many relative to the usefulness.
Same with any approach.