Summon Monster

[amethal]

Are there any rules, or guidelines, for assigning the level Summon Monster spell you need for summoning a particular monster?

Several books add new monsters to the standard lists, and I'd like to do the same myself, but I'd like to asssign them to the "correct" levels if possible.

 

[Infiniti2000]

Primarily it's based on CR or in some cases depends on certain abilities (e.g. lantern archon or other creatures with flight capabilities).