The method described by uofaengr and Josh at RazorEdgeknives gets you to where you need to go.
I prefer a slightly different scheme for FFG blades. I calculate the angular displacement between your knife’s two faces ahead of time. This gives me the centerline of the blade, which I record for all future sharpenings. Then, when I mount the blade in the jaws of the vise, I am looking for a zero-degree angle for the left face. This is very easy to attains, as the blade wants to settle there anyway. I kick the bottom of the right-hand vise jaw out by at least the total grind angle to ensure a positive clamp. Since I know the blade’s centerline angle, say 1.75 degrees, I know the blade is canted 1.75 degrees to the left and that value is exactly how much to deduct from the left angle and how much to add to the right angle.
The easiest way to measure the angle for your blade? Mount the knife in a vise (not necessarily your WE vise) with the blade entirely exposed. Place your AngleCube against one face. When you are satisfied that the reading is valid, zero the readout. Now move the AngleCube to the other face. The reading you get is the total displacement between the two faces – the primary grind angle. One half of that will be the angle of the centerline. No confusion, no arithmetic formulas. Straightforward and repeatable.
.
