Bob, most DIY roll film cameras use the film counter numbers imprinted on the backing paper, and a hole with a sliding cover to see the number as it come around. The position of the hole depends on weather your shoot 6x45, 6x6 or 6x9. If you want to shoot 6x7 your out of luck..
So how would the advance distance be figured for example the 6x17 below? Guess I could ask the builder, eh?