That would be the best method of raising Calcium levels.
However!! My only concern is the already high GH value. At 8dKH, adding more Ca will only raise it higher.
I'm thinking Sulawesi 7.5 has probably got more Magnesium in it than Ca, thus giving you that ratio. Mg will raise GH without altering KH.
It's not the right product for Cherries or Crystals.
It is perfect for Sulawesi though, as they like the high GH values of 8 or 9.
High GH values like that might cause moulting issues.
Might be time to bite the bullet and set that tub aside (unless you have sulawesi <-- in that case, pics!!)
This is the problem with premix products. It can't be altered.
If you want to do it cheap, buy CaSO4, MgSO4 and K2O (potassium sulphate) at Aquarium Online Store (AOS) our sponsor, and mix your own RO remineraliser. I have a post somewhere with the ratio I am using. This will cost you a fraction of the price of another tub of salty shrimp, and it will last you two lifetimes. Or you can sell the excess.
See if you get any other opinions, but I'm pretty sure you don't want GH that high. 5s - 6s would be best for Neocaridinas and Caridinas.