I've had a long "on again, off again" involvement with pinhole cameras.
Others on this list have already given you some good references. Let me
point out that you formula may be incorrect, or at least not "optimal". I
was taught to use
SQRT(focal length)
D = --------------------
28.4
Nancy Spencer and Eric Renner are co-owners of Pinhole Resource
(505-536-9942 http://www.yatcom.com/pinhole). They are probably the best
known source of pinhole info and products (and publish an interesting
magazine). Their list of pinhole diameters versus focal lengths seems to
use 27.1 instead of my 28.4 (not enough difference to matter). Your 44
will result in too small a pinhole. This will lead to extra diffraction
(fuzzy images) and longer exposure times. For a 75mm focal length,
Renner's formula would give you 0.32mm, my formula 0.30mm, and you formula
0.20mm.
A second less mentioned factor (important if you want to enlarge your
negatives) is how perfectly "round" the hole is. If you want a sharp
pinhole image you can purchase "laser drilled" pinholes from Renner or
Calumet, or make a few conventionaly yourself (using either wire guage
drills or sewing needles. Then examine all of them under a loupe and chose
the one that appears as a perfect circle. Good luck.
Tom Ferguson (tomf2468@pipeline.com)