I kind of like how Spirits were done. Magicx3. Call it "Astral To Material Threshold", the limit a living conduit is capable of channeling Astral(i.e. using magic). Anything beyond this limit, the conduit loses the spell/summon.
Virtue of this, especially if this gets added to magic splat book, is you could tie multiple magical things to it. Summoning? ATMTx3. Amp Ups? Can do ATMTx3 ramps (whether dmg, area, or some other form. Each would be tallied together. So at Magic 6, you couldn't do 18 dmg, 18 area increases, it would be more like 9 dmg increases, 9 area increases).
Could tie traits and meta-magics that modified that over it's base calculation of Magicx3. Could even go into negative ATMT metamagics that traded the ATMT for say, guaranteed drain resistance ("By buffering yourself against the negative physical effects of Drain, you are able to draw upon less of the Astral" [ATMT is halved, mage reduces any drain by 1/4 of drain resistance die pool before rolling to resist drain]. I see a lot of interesting trade offs that could be made by making a calculated magical attribute such as this.