I guess the answer to that is rather than placing it at whatever point, you bring the rifle up to sight it and have someone slide the sight along the barrel to where it is clear to your eyes. That is where the sight belongs.
With that being said, I think I recall that I heard somewhere that the rear sight had to be located at least 6" ahead of the breachplug in order to be legal to shoot in some matches. And of course, not to have it at the balance point where it could dig into your hand when carrying it.