looking at the code, the line that does the accuracy adjustment is in the middle of the various spaghetti code. (It is not something I had looked at, ever, so is likely to be in its original place!). Various adjustments for movement and target size etc came after the percentage divider.
In my test scenario, US bazookas were opening up at about 14% shot 1 hit chance. Cranking their preference setting to 30% to-hit only lowered that to 10-11%.
So I commented out that line, copied it to the
end of the accuracy-determination spaghetti, and the to-hits went to 6-7% first-shot. Not 30%, but these numbers get modified elsewhere in the spaghetti, by things like leader ratings and all that jazz, so determining precise values, even with the code to hand, is nigh impossible. (And we still get folks who want the "tables"

).
Note to self - if I ever write a wargame engine from the ground-up, make it table based, just like normal tabletop rules!.
Solution to make the code behave as end users would expect, is to move that line to where I did, it seems.
Good catch! - something we all had assumed was working in the original SSI code, was not.
Cheers
Andy