As you get into really wide angles, it's amazing how many bits of mechanics can obstruct the light path. A pinhole is strictly a ray tracing situation, there is no retrofocus or other tricks like might be done with lenses.

In another thread, one about converting a Brownie camera to a pinhole, I posted a diagram of how one might do a scale layout to determine what will or won't vignette. It was tailored to a 6x6 format, but the method could be adapted to any situation. The biggest problem is making accurate measurements if there is a an aperture at the bottom of a hole or some other stuff in the way of using normal measuring tools.