I don't know all the theory, but I made a makeshift pinhole out of an old 35mm Fujica and some cardstock last week.
Focal length was 45mm, and the pinhole ended up being 1mm ... so I guess that's f/45 ... and the results were much too blurry for my liking. I took the camera out for a test without measuring the hole, which was foolish indeed.
This was the best result from my test:
I've since made a second attempt at a pinhole which seems to be around 0.28mm which hopefully will give much better results.
No idea if the problem here is absolute or relative size, though - I think f/45 is waaay to big for a pinhole regardless the absolute size that works out to be.