with material removal in the thousands you can model the volume removed by milling as a small cylinder.

bore^2/4*(amount milled)*3.1416 this works pretty good for b16a heads and other round chamber heads it is somewhat less accurate for gsr heads but still good enough.

Tell me what compression you want to achieve and I'll tell you how much to mill. BTW as mentioned earlier for every .012" milled you retard the cams 1 degree each.