A good way to measure shoulder width is to measure armhole to armhole across the back of a suit jacket THAT FITS. If you are as oddly proportioned as you say, and don't have your clothes custom made, it s likely, however, your suits don't fit very well. Not much help there. Nevertheless the jacket gives the person measuring some sense of where to measure. Try it.

But others did suggest rightly. It is not about rules and appearances but rather about comfort and easy breathing. I assume your arms must be fairly long. Over that length the angle from your shoulders to the bar must be fairly small. Surely a 38cm bar can't be far off.