They have just mentioned - (Dupont 1992)
They have mentioned - The solubility products for sparingly soluble salts are based on expressions derived from published data on solubility as a function of the ionic strength of the solution at a given temperature. The equation for calculating Ksp for CaSO4 at 25 Degree C.
And when I checked in Dupont document for Ksp, they have given a graph, it seems it must have been derived from that.
So, still the question remains is that how to calculate Ksp of CaSO4, BaSO4, SrSO4 at any given temp.? - Can I apply Van't Hoff equation here as well???