For this, you can add "flexibility" magnitudes as multiple canon spells do. Personally I'd go with +4 for a broad major focus (birds), +3 for little major focus like (birds living during day), +2 for minor focus like (birds of prey), +1 for a couple of possibilities (hmmingbird, ibis, eagle).
But, a true MuVi specialist would not have any MuVi formulaics because of that limitation of "flexilibity". He would have all his OTHER spells formulaics. Then use the MuVi spontaneous (with focus and all bonuses he can get) to do whatever he wants.
As for the total change with boost power, I have gone with /2 , -3 magnitude, and a Creo requisite. I gave it +2 magnitudes boost of power
So for example (from memories so I may do mistakes, not having books):
Mu(Cr)Vi 50
R: touch
Allow you to transform a spell up to level 15 (base 45-3 magnitude, /2) to any spell up to level 25. Penetration remains unchanged but you can reduce the target spell level, and the difference is added to initial penetration.
More requires ritual power (level 50 limit).
Once a MuVi specialist is able to spontaneously cast this, he can say "I'm a mutovim expert". Not before.