Hey Brian, I do what you are doing right now for a living and you have the right idea. Unfortunately there is no mathematical equation for you situation that I know about because if your cameras were not synced up (jam sync, ran through one deck, etc.) then you can’t really calculate this. You would need to find out how many frames you are off and in which direction and then account for the drop frame camera’s timecode. Seems to me like what you are doing (grouping by in points) is the best way to my knowledge. It’s time consuming and sucks, but the most reliable way.
Frankie