When I bought my first fancy knife from a local kitchen supply store, they taped a folded post-it note to the box and showed me how to make one. Fold a piece of paper diagonally, one corner to the opposite corner, to make a 45 degree angle. Then fold it in half again by folding one long side of the triangle to the opposite long side. You'll now have a 22.5° angle to use as a visual guide to get you close enough.