I think part of the CR problem is that the span of abilities/capabilities you will see in various PCs will vary enormously.
Depending on whether the players are intimately familiar with high level play, whether they use all the min/max cheese that is available (multiple archer PrC's, archmage/red wizards/shadpw adepts, clerics with hospitaler/windwalker/hexer PrC, paladin/divine Might/divine shield/shield bash, and so on), whether the characters actually had to survive to attain those levels , or were created at 23rd level.
Thus pinning done an exact CR gets harder and harder, the more elevated the level.
Sitting here, looking at the book, I can only say, that this is summon vermin ability is another ELH oversight, because it isn't clear at all. I would assume that it means per caster level. Given that this ability is always at caster level 20, why didn't they just say 200HD of vermin?