CR of lone Eidolon

[Raef13]

Hey guys so I was wondering if there is a general rule to the CR of a Eidolon that is away from its summoner (possibly buffed, not sure)? Something like Eidolon CR = 3/4 Summoner level. Or maybe 1/2 level. I have the idea for a recurring villain using the eidolon and I think it may work fairly well.

[Dasrak]

There are rules for this. It's based on the Eidolon's outsider hit dice according to the creature hit dice table in the monster creation guidelines. The rule comes from the Unfettered Eidolon bestiary entry.

Unfettered Eidolon wrote:

An unfettered eidolon's CR increases with its Hit Dice using the "outsider" line.

So, for example, a 10 HD Eidolon is CR 8.