I need a better formula.

I have two lists (that change frequently) one is up to 50 items long, and
the other is up to 5000 items long. Both are in columns. Many of the items on
the 5000-item list are on the 50-item list, but not all of them. I need to
know the arrary position of the last item on the 5000-item list that is a
member of the 50-item list. So if there are currently 750 items on the long
list, but the last 9 are not on the short list, I need the formula to return
741.

The way I'm doing it now works, but causes, "Not enough system resources to
display completely" and that's causing problems. Currently, I'm using a
helper column that is 5000 rows long. All it does is look at the
corresponding spot on the long list and outputs row() if that spot is on the
short list (I'm using SUMPRODUCT) otherwise 0, then I look at the MAX() of
the helper column.

I appreciate your ideas!
Thanks,