Remember that if you scale it based on magic level, then the max hit will increase with imbued heart/magic potion. Basing it off magic level is a poor idea. The hp requirement would more than fix this. Say 75 hp/mage requirement and 25 prayer to keep the staff is still 58 combat, a common build at 58 combat is granite maul pure (50 att, 70 str, 31 pray, and 63 hp) - which can hit 29-29-29 with gmaul. So while the staff would 1 hit people at that bracket, the gmaul already can do the same.
Plus at 58 combat some people have smite, so you would definitely be risking the staff if you wanted to pk at that bracket with it and only 25 prayer.