Add `--update-refs` to your interactive rebase and it will give you an easy line to know how many commits to drop because it will add an `update-ref` line for the old branch. You can just easily delete everything up to and including that `update-ref` line and don't have to manually pull up a git log of the other branch to remember which commits already merged.
(Plus, of course, if you have multiple branches stacked, `--update-refs` makes it easier to update all of them if you start from the outermost branch.)