Arguably there is no "right" focal length, but loosely speaking, a focal length equal to the film diagonal is a "normal" angle of view. At extremely wide angles of view, the pinhole -- even if 'perfect" -- becomes a narrow ellipse viewed from toward the edges and you get significant light fall-off. The numbers for my 4x5 referenced above worked out to about 104º angle of view; there's some fall-off but it's tolerable. With a program like Pinhole Designer you can play around with the parameters and see what looks interesting.
Originally Posted by Alexander S.
In general you set a couple of parameters and calculate the remaining number. For example, pick a film format and pick a pinhole-to-film distance based on angle of view, then calculate the optimum pinhole. Or if you inherited a pinhole plate, you might use that size and a film format to pick an optimum pinhole-to-film distance, etc.