Here's the thing.....Shadows of the Warp sets the precedent here. Shadows causes a test on 3D6 with anything over 11 a fail but only perils on double 1's or 6's.

Runes states that anything over 12 is a Perils. So basically you just treat anything over a 12 as double 6's.

Also, it bothers me that you guys would want to make the best anti-psychic ability in the game even MORE broken with this line of reasoning. You can't have your cake and eat it too!