No particular formula that I know of, and how much the CR will be raised for any given cut will depend on chamber volume and total cubic inches. So it will vary from one motor to another. However, a rule of thumb that will get you kinda close most of the time is one half a point for every .030.
You can typically cut .030 without having to cut the intake surface (unless you're really a perfectionist), but more generally requires a cut on either the intake surface of the heads or on the intake itself. Also, front and rear sealing surfaces of the intake/block interface will need clearancing (on a small block), although substituting RTV for the end gaskets should suffice.
But, you want to go from 9:1 to 10.5:1, which will require close to a .100 cut. I'm not an expert on magnum heads, but I'm thinking they won't take that big a cut. Plus, now we're talking about potential valve to piston clearance issues, as well as rocker arm geometry issues. Probably best to change pistons.
