Yes, you hold the knife at your desired angle perpendicular to the belt.

The wear marks down the middle of the belt are do to the fact that you're grinding on a slack belt. When you put pressure on the belt the main part of the belt that is touching the workpiece is the center. It's also why you'll find sometimes that the area closest to the ricasso gets a little less sharp, because it's in contact with the abrasive less so on a knife with no choil, I spend a tiny bit of extra time grinding that. Let me know if you have any other questions.