Oh, I just thought you could use the shape to calculate where to place the stop. BTW, how is it shifting poorly? If noisy, I find many times the cage on the NR is tweaked and the jockey wheels not aligned with the cog, chainline. Once you've squared the dropouts and hanger of course...

Chain length good?

Is it you don't want to modify the Simplex dropout that you don't tap it and file it?

Honestly, it doesn't take much to get a friction rear derailler to move side to side and change gears. The stop only seems to keep the derailler from rotating too far forward in the highest gear. In most gears, there's no contact.