It is suggested (by the DMG) to account for defensive options by assuming increased AC, e.g. +2 AC for magic resistance.
I would like to give some of my monsters the spell mirror image as part of a spellcasting / innate spellcasting trait. There is precedence for this: the Alhoon, the Faerie Dragon, and the Lamia have it, for example.
What would be a rough estimate of the AC equivalent for mirror image for the purpose of determining CR / combat performance?
Tangentially related (compares mirror image to other spells for PCs concerning the defensive capabilities): What's my most efficient use of spell slots to help my AC?