Some times it takes a while for me to see the nose on the end of my face. As I was re-reading Saw Filing – A Beginner’s Primer (Vintage Saws Library), I noticed:
Some people actually make a jig which consists of a ¾” by ¾” square piece of stock. In the center, saw a slot which is 25 degrees to the right of perpendicular. Saw to a depth of about ¼”. Flip the stick over, and file a slot 25 degrees to the left of perpendicular. Finally, place the jig on the saw blade.