Please don't consider this as spam as I am just trying to help you out:
Defence means EVERYTHING when mage boxing.
I have a 99hp 99 mage 90 defence mber and I win about 70% of the fights just by mbing people with 80 or less defence.
While when I fight some1 with 99 defence I mostly lose.
Anyways in my experience having around 90 is best, because that way people with lower defence will still be staking you, 99 defence just scares them away.