It wouldn't be simplier to change the strenght of the summon based on the level of the evocher? I mean at level 1 a weak undead\animal\spirit at level 3 an average summon, at higher levels more powerful summons.
Just like in OS II with summoning lower than 10 the incarnate has normal stats that increase by a 10% for point in the skill, when summoning reaches 10 points the incarnate evolves in champion incarnate that has higher stats.
Oh wait, just did a little of research and found that the Summon Undead spell has different levels from I to V with their own list of level 1 to 5 summons.
Oh, again

, I see that the original post has a "no scale" tag. And I think that what I just wrote is scaling of the spell. No, no idea of what to do.