Thank you all for your useful tips and links!
As far as I understand, the 45mm is actually a bit wide for 120 film; that is to get a good image quality over the whole surface of the film. (I didn't really choose 45mm, that's what mrpinhole.com gave me when I typed the film dimensions. But such a wide angle looked pretty nice at first sight.) So, the 'coverage' value is quite important it seems to make sure I get a constant image quality.
Also, the regularity of the hole will have an influence on the quality of the image at the edges, right? As I'm going to make the hole by hand (read: no precision tools here), I guess I'd better take a narrower angle and a larger coverage.
So for example, if I take 85mm as focal length, the coverage would be 163.2. That would produce an image that would be large enough for my film, right?
Also, I'm wondering what would be the diameter of the "useful" image for a given hole size. I mean: for a given hole size, there should be a circle in which the image is sufficiently good. And I expect the "edges" of the image diameter to be of less quality (blurred, lighter...).
Is there a formula to calculate that too?