Btw, old reply... but using two head gaskets does *not* increase CR, using an extra gasket would raise the head, thus increasing the volume available at TDC, thus reducing the compression ratio.

This is not my forté really, but as I understand it, if you have an 1800cc engine, making each cylinder 450cc, for 8.2:1 CR you'd want the volume of the chamber at TDC to be 54cc ?

Raising the head by a millimeter, might add 1-2 cc of volume, thus lowering the compression, example, lets say 1 mm raises increases it by 2cc, that gives you 56cc of volume, meaning a compression ratio of 450cc / 56cc = 8.03:1 CR.

(Perhaps I should add to the equation the fact that a more volume at TDC would also result in more total volume, but relatively the 2cc change is much more drastic for the TDC volume, so even if we'd add the 2cc to the total volume, 452cc / 56cc we'll get the reduced CR of 8.07:1)

This might be an oversimplification, but it should reveal the obvious, raising the head increases volume at TDC, thus reducing the compression ratio.