Like Rich, I think you will find there is no "rule" as to where the transition must fall, especially on American work.
If you know what your going to build draw it out on paper and see where the transition looks best in reference to your other components. Could be you like it over the entry pipe. Maybe it would look best a little behind.
Since you are apparently custom ordering the barrel, you can have it placed where ever you find it fits the your design.
Personally I'd be more concerned about how the maker makes the transition look and whether or not he kept the barrel light enough in the round section. Too many barrel makers make barrels with funky, unattractive wedding rings that don't look true to period work and need a lot of file work after you get it. And almost all of them leave way too much metal in the round section. Fine for a buck and ball gun, but what I've observed, original fowler barrels have walls that are generally pretty thin out front.....very, very thin by modern scattergun standards that most have become accustomed to.
Enjoy, J.D.
