I've now had twice the Mists of chaos corruption which should cause Pe-10, but the actual modifier was -11. (Dunno if it's even bigger, since both times -11 was enough to drop it to 1.)

Also, now I have both Mists of chaos and the Unified eye corruptions, and the total modifier should be -6, but it is -12 and again dropping me to Pe:1. Is the Mists of chaos modifier a lot bigger than it actually states?