I'm not sure why you chose a focal length of 45mm ..? That would be equal to a 45mm - wide angle lens for a 6x6 or 6x7 format. The limiting factor here would be "coverage" - with a 45mm focal length that would be 86.4mm (roughly - there usually is a large amount of "fall-off" - uneven exposure at the extremes of the frame) and 6x7 would probably be the largest acceptable format.
If you choose a longer focal length, the optimal pinhole diameter will be larger, and the circle of coverage larger - and the "angle of view" smaller... as it would be with a conventional lens.
"Mr. Pinhole" is a good, useful program. I would also suggest "Pinhole Designer" available from
[ http://www.pinhole.cz ]. LOTS of good information there... also, mention of a "Pinhole Enlarger ..."!!!
Film advance and framing is a problem. Many will "savage" a camera by removing its lens and substituting a pinhole, either through a "body cap" or with a cardboard/ paper mache', -- I don't know -- silly putty - jury rig.
I'm in the process of building a pinhole camera right now ... using a metal Maxwell House coffee can as a body. I'm not going to worry about precise framing - I'll just cut a length of film and tape it to the bottom of the can.
Should give me a focal length of 6" and an aperture of f/300, with a pinhole diameter of 0.020"
BTW..., WELCOME!! to APUG.