Measuring your span on a drilled ball?

[bowl1820]

but in the case of buying a used ball with grips i'd compare cut to cut so that when i replace inserts the span should be right. Correct? ie Switch Grip outer sleeve to finger cut correct?

Yes, Given if the cut to cut measurements of the used ball are the same as your other ball. You should be able to just swap out grips, of course the used ball might have different pitches than your other balls.