Just a thought that came to mind, if one’s glans is larger than normal when compared to the shaft just behind, that could also cause issues with getting the size correct. I suspect that if that’s the case, measure the widest part of the glans and perhaps take the difference between that, and the size that you get for the shaft, and use that as the size for the innerwear.
(larger size-smaller size)=difference
(difference/2+smaller size)=innerwear size.
This will of course vary with the difference between the shaft size, and the glans size. If the glans is smaller than the shaft, then subtract the smaller from the larger, and follow the rest of the math. Be sure to adjust as needed, for comfort, fit, safety, etc.