I could not find the the algorithm by which pymol calculates the solvent ASA.
What I understood is that pymol adds the vdw radii of atom and probe, makes the sphere around the center of selected atoms, puts some dots and according to the density of dots it calculates the SASA.
or it simply uses the Richard & Lee algorithm of rolling water.
Please clarify my doubts.
Best regards
Neshatul Haque 
PhD, Dpt of Biotechnology
university of hyderabad