I'm not sure how the program computes coverage. On my calculator I just verified all the other numbers... if I assume a "daylight" wavelength of 550nm and a "user constant" of 1.8 it's very close to a 0.4mm pinhole.

That angle of view will be similar to a 24mm lens on 35mm film. Which is pretty nice, I think. ( I'm biased... 24mm is my favorite lens )

Since I use paper negatives, I usually use 500nm or even 475nm for the wavelength, because paper is so sensitive to blue light. And I tend to use a smaller "user constant" of 1.56. If I was making that camera for paper, I'd start with a 0.33mm pinhole. But I think your 0.4mm will probably work well for film. If you will be mostly using paper, I think you could get away with a smaller pinhole.