[Sorcery] Point value of allies with Sorcery

[VariousRen]

My campaign is using Sorcery, and one of the players would like to create a spell that summons a horse for them to ride. They've built the horse as an ally, but that ties the speed of the horse (its points are all spent on extra speed, because that's all it's used for) to the casters point value.

I would like to have the point value of the ally based on some constant value so that a faster phantom steed is a larger spell, instead of just requiring a higher point character.

Is there any way to link allies to a constant point value in sorcery?

[Leynok]

One thing that I've done in the past is instead of improving the Allies alongside the character, they remain the same as when you purchased them and when they're weak enough to fit under a lower power category, you get some points back.
Say you're a 100 point character with a 150% Ally worth 10 points. Once you reach 150 points it becomes a 100% Ally, giving you 5 points back, then as you reach 200 points your Ally is now a 75% Ally so you get another 2 points refunded and so on.