If you measured 25mm wide open shutter, then your f/# is
300 / 25 = f/12
Which is ~2 stops down from what you metered for right there. That's not quite the working f/#. Magnification due to object distance plays a role as well. Additionally, positioning the shutter as-is will cause a change in magnification of the entrance pupil and changing the f/# as well. Going back through, pupil magnification sounds like 2x. Those effects could add up to an additional stop or two.
Working f/# ~ (1 + m/P) * f/#
m = d(object) / d(image)
P = 2
For object 2m distant, f/# will be about 1.1 x 12 ~ 13.2
There's also a vignetting effect due to the shutter position so far behind. Sort of like a T-stop effect but in reverse. If I'm calculating correctly, that could be 2-4 stops depending on what the original f/# is.
So you could be 4-6 stops down from all those effects.
That negative also looks very dense, as in it looks very overexposed. However, closer inspection of the edges indicate significant fogging. To overcome that fogging requires additional exposure..conversely exposing as you did will not get the image very out of the fog and it will look underexposed. Maybe another stop or two from that effect? Veiling glare due to light leaks could account for another stop. 2-3 stops from those effects.
Also, developing HP5+ at 200 (N-1) in Rodinal 1:50 at 20C should be more like 7:00 with 30s agitation. N-2 is about 5 minutes, so you're 1 stop underdeveloped as well. Rodinal 1:25 would be about 6:15.
So I'm estimating 6-9 stops down!
Conclusion: death by 1000 cuts. I agree with Vaughn..You need to refine your "no budget" camera, verify developing times, and probably buy fresh film!
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