Under the box lid of a Sears Craftsman 45° miter cut-n-clamp set, which was produced in the 1960s.

1. Measure the width of the picture. (X in inches)
2. Measure the width of the molding. (Y in inches)
3. 3 Add twice the molding width (2Y) to the picture size (X) then subtract 0.5.
4. This will give you the length to cut your molding. (L in inches)

Then you can do the same thing for the opposite dimension if your picture isn’t a square.