Place the tinfoil on one of the steps, sit down on it, lift your feet and rock back and forth a little. Try and find the center of each divot made by your sitbones, and measure the length between the two. Use the narrowest seat you can, while still being wider than that measurement. Make sure not to wear jeans, or something thick.

So if you measure 125mm, use a 133, if you measure a 136, use a 143, etc.