Rather than referring to the amount that is taken off, all should be referenced to the standard SST60 head which has a combustion chamber depth of 0.450 in and allowable limits of +or- 0.010. When I use to race the modified 56ci had two heads that measured 0.400 and 0.380. These had compression over 200 psi but trying to remember 20 years ago is a bit difficult. I think it was around 220 psi. While the .400 head was fine the .380 started to crack and warp.

Stephen