In discussions with some in the industry, I was led to believe that the industry standard opening was 3mm, but many artisans are moving to 5mm.
The 3/16 tubing he uses is the equivalent of 0.1875 inches or 4.7625 mm. So, that is a fairly generous draw. Cobs use similar sized openings, so I'm not surprised that it has worked out for you.
BTW, I did contact Paulson, as well as another member here who makes pipes. I'm weighing the pros and cons of buying a facatory made Savinelli Punto Oro Sandblasted Silver Army Mount (310 KS) or commissioning a design.
Paulson was very responsive. He got back to me within a few hours of my inquiry.