The only "rule" I know when it comes to toe-in is that you should never toe-in any further than directly at your head.

My M22s are toed-in about midway between straight ahead and directly at my ears. I found this gave me the best mix of imaging and soundstage width.

I read another post where someone used the wedge shape of the Axiom cabinet as a guide and toed them in just far enough so that the inner sides are parallel to their line of sight. This method makes it easy to see if the angle of both speakers is the same.

Experiment, experiment! Only your ears can tell you what works best in your room.