Shaving Magnum Heads

I am thinking about while I have everything apart of having my local machine shop shave some meat off my 318 magnum heads and go to a aftermarket thinner headgasket to gain some compression.

Has anyone shaved stock maggie heads any estimates on how much I should take off to gain .5 a point or so?

Compression is one aspect of engine building that I still do not fully grasp if I am asking something stupid sorry lol.