If you want to go crazy with the math, here is a thread to read all the way through:
http://www.f295.org/main/showthread....posure-mapping

Some months ago I started to use these formulae to try to find the shape of a surface that would have constant illumination ( instead of a flat film surface ). The shape accounts for the off-axis squint, the "tunnel" caused by the thickness of the pinhole material, and the distance to the film. I'm not convinced it really takes into account the shape of the diffraction blurring all that well. I haven't gotten back to it but the shape is somewhat vase-like, at first bulging outward and then diving inward toward the pinhole itself. It makes sense: as you move away from the center of the image, at first the distance is the dominant effect, requiring you to move upward closer to the pinhole to compensate, but as you move upward, the squint becomes more pronounced causing the radius to decrease.

( My undergrad degree was mathematics, and I've been a member of the Mathematical Association of America for almost 30 years now... so this sort of thing is a great temptation for me... )

But. Recently I've had to catch myself making things too technical and complicated. So I pulled back from all of this. I don't want to mess up the simple joy of finding the picture, placing my pinhole coffee can or cardboard box, and then watching the magic of the image coming up in the developer. To me the idea that the light from the scene landed directly onto the paper to make a permanent captured image is just wonderful. So these days I'm trying to keep things more about "play" and less about the technical part. Not that the technical part can't be fun, but there is a balance to reach!

So my advice: build the camera and have lots of fun making wonderful pinhole pictures with it!