There is a simple forumula that goes something like for every .006 milled from a head, combustion volume drops by 1cc. I think it is .008 inch off per cc. I had my 113s with 72 cc chambers milled .026. New volume was 69 and 69.5.
Prior to that I spoke to Mummert and he said he had milled heads as much as .040. He also said that between decking and head work, if .040 was exceeded, then manifold fit could become a problem.