However, magic is controled not as much for its damage, but for two other factors.
The first is a global one called character speed. Nu Mous has inherent low speeds which result in less turns and therefore less dps per round. Only one nu mou job has somewhat decent speed iirc
The second is on most magic departments, and is MP. mana can become ignorable thou, via blood price or clan privileges, and other means like potions. Still, in a game in which characters starts at 0MP and gain 10 per turn, a spell that costs technically 2.4 turns in MP gain has to worth the waiting versus three times casting a basic spell.
As sidenote, aqua and aero are usually treated as tier 1.5. Their MP cost is closer than T2 than 1, so I'd stick with x1 multiplier for them, at the very least.
In any case, on first read, I like your approach