even if you do not know the exact calculations
you could remove the back of your box camera and put a piece of
parchment paper, velum, wax paper inside and see where things focus.
My camera actually has a ground glass. My problem was that to adjust the "bellows extension", I was going to have to cut some material off. So I wanted to know beforehand how much to take off. I ended up using my dremel to route 1-2 ish milimeters off. Even though I have a ground glass, it's still very hard to see exact focal distance at f/6.8 on a 90mm lens. I shot some tests shots and will enlarge them to 8x10 or so to determine where my focus point ended up.