What you're planning sounds good to me. I would have said make a sanding block that has a face square to the bench, block up the wing to the proper height and sand. Your way is basically the same except instead of moving the block against the wing you're moving the wing against the block.

By the way, symmetry is more important than absolute accuracy. In other words it's more important that both sides of the wing are the same than it is that they're exactly the right angle.