My heads were milled-effect on compression ratio?

My heads were milled-what effect on the compression ratio would it be-they are the proverbial "x" heads and this is the first time they were cleaned up with .031" taken off both. I understand there are variables involved here and have seen where machine shops can "cc" the heads but didn't have that done. I plan on using the good Fel Pro blue head gaskets(Perma Torque) because I've never had trouble with them . I believe they are .040" or so, and don't recall what factory steel shim style ones are, to throw that into the equation.