Brad
Not sure if you have figured it out but you actually have two angle to hold when sharpening the corner chisel....the first is obvious, the angle on the face of the chisel. The other is esentially the same angle but on the other face.
Put the chisel on the left side of your stone holding the handle about 25 degrees up from the plane of the stone. Now also move the handle out to the left about 25 degreesfrom the edge of the stone. The other edge is of course is done on the other side of the stone.

A couple of strokes on one side then the other will lessen the chances of scoring one face deep which you would have to work out...then lap the back holding it FLAT to the stone. If you put any angle to the back it will cut out into the mortice rather than cutting straight down.

At least that is how I have done it with good results.

Bob