When they sell pistons they list the piston dome + or dish - , they are not selling you compression, that is what the calculator (and your heads) determines. They have no idea what your head volume is nor do they care. That's probably why they sell pistons with the - dish and the + dome and the compression calculator asks for this value inverted as it is determining compression.
^^ my stroker cast pistons were so mismatched (MP castings by ?) that I had to take weight out the pins to get the heaviest one down to the 2nd lightest one. I actually added a zip of MiG wire to the lightest one inside the pin to get 3 grams. They were all over the place, maybe even 2 batches.
