>>3261449>>3261409For 8x10 the shortest focal length you can have is about 135mm(5.314") which makes the image diameter 10.1969". Just at that distance the hole needs to be about half a millimeter in size. It'll have an F-stop of f/256. That's a 3 second exposure with 100ISO film on a sunny day. Which will be a pretty long exposure for anything indoors.
Personally, I'd go with a slightly longer focal length so the margin of error in aligning the image diameter on the paper isn't a problem. If the pinhole is the correct size for the focal length then perhaps there's something different with the paper that needs to be factored into this? The stuff I'm reading online says that indoors can be 30 minutes or longer for the exposure:
https://www.lilblueboo.com/2013/08/pinhole-photography-taking-a-photo-exposure-times.html 