read this in over rev magazine.

compression ratio=(gv+dv+hv-vv+pv) / (gv+dv+hv-vv)

gv = gasket volume
dv = deck volume(positive for pistons that are"in the hole",and negative for ones that protrude)
hv = head volume
vv = dish/valve pocket/dome volume(negative for dish and valve pockets,positive for dome)
pv = volume displaced by piston

quote
"if you know what your target compression ratio is.when ordering custom pistons you can adjust the dome or dish and compression height to get exactly what you want instead of messing with different gasket thicknesses or milling the head."

just thought i would share,i still trying to figure it out.

they also say there is a calculator at http://www.rosspistons.com/calculator.php they said "it will do all the heavy lifting for you" i just went to the link,and it it very interesting