I'm not 100% why I was getting the error. What happened was I created
the formula, which had a logic error. That is, it never displayed an
error, but the output was unexpected. I copied that formula to all the
rows below...

When I edit a rows forumla with your correct version, it gave the
error.. something about an inconsistant formula. I then copied that
formula to all the rows again, and it was fine. Was it reporting that
this row (that I had just edited) wasn't the same as all the other
rows, and because I had copied them... Excel was just warning me that
it wasn't the same as all the copied cells?

(Hope that's understanable... :)