When I got my letter of acceptance from Mac, they said I was guaranteed residence and had to fill out the deposit. I did everything and submitted the application before the deadline.

What happens if my final average after exams drops below 81%? Does my residence guarantee get revoked? And if it does, do I get placed in the bottom of the waiting list?

It does not get revoked. If you got your application in, and pay the deposit by the deadline, then you're in!

Thanks! Guess I can stop looking for off campus housing then

if it falls below the condition for your program then you dont get in the school at all.