Find a flat headed object that will fit in the shoe. I used a glue-stick. Stick the piece of paper to the wall with tape. Place the flat headed object next to the wall. Mark the top of the object on the paper with a pen.
Place the object in the shoe. You will need something thin to cover the distance of shoe to wall. Something like a bank card would be fine, but mine is bent so I used a ruler. Mark the top of the object with a pen.
If you make the marking from the top of the thin object, take the object's thickness off the final measurement. My ruler had a 2mm thickness, so I took 2mm off.