The shape of the penis (determined by some of the penis flags, like: blunt, flared, and tapered) will also affect the volume of the penis (though not knotted for some reason). If the "effective" volume is checked, then the lubricated and sticky flags will also factor in to whether a penis will fit.
But yes, the volume pretty much starts off as a volume of a cylinder plus the volume of the tip's shape. The default tip is a hemisphere, blunt is a cylinder tip, flared will be a 1.3 multiplier to whatever the tip is, and tapered is reduced to 75% of both cylinder and tip. For the effective volume, lubricated will make it 25% easier to fit and sticky will make it 25% more difficult to fit.
Hope this helps.