Just clamp the blade flush to the inside faces of the clamp's two halves, which guarantees a lot of surface area in contact and grips the blade more securely. That means the clamp's two sides will angle inward, toward the edge of the blade; not a big deal. That's far more important than trying to keep the clamp's two halves exactly parallel to each other (advised by many, but it's a bad idea with FFG blades). Trying to keep the clamp's sides parallel, on a blade grind that isn't parallel, creates a bigger problem by minimizing the 'grip' afforded by the clamp, and that'll allow the blade to move. A blade moving in the clamp will negate all the advantages of a guided setup, and one might as well not use the clamp at all, in that event. Much better to use the clamp to keep the blade from moving, which will result in a very crisp edge.
David