Fundamentally, it produces the correct result for what inputs there are...... but there aren’t quite enough.

No input for gasket bore(so I have to assume it’s using the same as the cylinder bore), and it doesn’t take into account the area above the rings.

Modeling a 446 with dome pistons down the hole .010 and a 4.41 bore gasket that’s .045 thick, my staples calculator and the Summit calculator differed by .15:1 ....... the summit one being higher, since there was no input for the bigger gasket bore or the area above the rings.

If I remove the volume for the area above the rings and make the gasket bore the same as the cylinder bore, the result is within .02:1(which is likely from a difference in how far out the decimal point is taken for each entry).

Edit- it does appear there is a slight error with what’s its using for a conversion from CI to CC though.
4.35 x 3.75 x 8 cyls is 445.85ci..... which it has as correct.
But the conversion to CC is 16.387...... which is 7306.14cc.
It’s showing the displacement cc as 7309.02(16.393 conversion).