Many of your equations are here: https://www.kennethleegallery.com/html/tech/bellows.php
So, with 40x50", a 4x life size image will give you a subject size of 1/4 your plate size, or 12.5x10", which would be a pretty good tight head shot not unlike the example in your first post.
Let's stick with inches to make life easier. You've had a 24" (600mm) and 36" (900mm) lens recommended, so we'll do the math with either of them. Common lenses in those focal lengths are Tessar formulas, where the image circle is just a bit larger than the distance of the lens to the image plane, so we'll stick with that math. I.e.: A 24" lens focused at infinity will have a 24" image circle, and you need a 64" image circle, so that won't work.
OK, 4x life size:
M = (B-F) / F
where M = magnification ratio, B = bellows extension, F = focal length
4 = (B-24)/24, solve for B. That's 72" of bellows extension, giving about a 72" image circle (and many stops less light), so a 24" lens would work just fine. 72" is 1.83m for most of the world!
4 = (B-26)/36, that's 108" of bellows draw. That's a pretty large camera, if you want to work with a 900mm/36" lens!
What's your working distance with a 24" lens and 72" of bellows, or a 36" lens with 108" of bellows?
1/F = 1/D + 1/B
where F = focal length, D = distance, B = Bellows Draw
1/24 = 1/D + 1/72, D is 36" (just under a meter in the modern world). That may be a bit close to your subject, or not, that's for you to decide.
1/36 = 1/D + 1/108, D is 54".
Maybe try to find a 800mm or 900mm lens? Good luck!
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