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3 basic variables to work with, Volume of cylinder (Vcyl), volume of everything above that (Vtdc), and Compression ratio (CR). Example: for a 4.31 bore, 4.15 stroke, 10.0 comp ratio, Vcyl= 992cc, the unknown variable is Vtdc, the general equation reduces to 992= 9.0 x Vtdc; so Vtdc= 110.2. To increase the comp ration by .5, the equation changes to 992=9.5 x Vtdc, so Vtdc= 104.4, a reduction of 5.8cc. Milling a hemi chamber is about .0041"/cc, so 5.8 x .0041= .024".
All hemi heads have a register on the deck near the exh face at the center of head (about .5" x .5"), once the register is gone, that is the factory safety limit for milling (usually about .080 deep when untouched.)

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