I am also seeing a big variance between the Butler and Wallace Racing compression calculators. I like the idea of running the 15cc Icon piston that you shared. However, the Wallace calculator shows 10.55:1 while the Butler calculator shows 9.65:1 with all of th same information. I also spoke directly with Sealed Power about their L2262F .030 over piston, and their "specialist" told me that I would be at 9.98:1 with the 72cc heads and this forged flat top piston. It seems like the Sealed Power specialist and the Butler calculator provided very similar information but the Wallace calculator is almost a full point higher. I entered 4.150 bore, 3.75 stroke, 72cc heads, 15cc dish, .041 gasket, and .000 deck height on both calculators. Am I missing something here?