Well, it's a bit crude, but there was the earlier suggestion of 12+HD/2 (round up?) + relevant ability bonus (min 0) of the base undead.
Examples:
Human Zombie (2HD, Wis 10, Cha 1) gives shades with Wis 13, Cha 13.
Human Skeleton same but only 1HD.
Vampire Spawn (4HD, Wis 13, Cha 14) gives shades with Wis 15, Cha 16.
etc.
I guess I'd make the whole "canopic template" including the shades a CR+2, but I'm still not comfortable with the more intelligent undead since they lose their strategy. Maybe we should say they use their special abilities (like dominate for the vampire or spawn for example) at an instinctive level.