StatCast allows researchers to pinpoint the location of the catcher’s right shoulder behind the tip of the plate. Tom Tango uses that data to show how a catcher moving closer to the plate improves pitch framing but also increases catcher interference calls:

So you can see why catchers would try to nudge their way closer to the plate, as each inch is giving his team a run in framing. But you can also see how each inch is adding interference calls, and each interference call is worth 0.3 runs (plus potential injury). Being a catcher is a very tough job.

TangoTiger.com

Sixty six behind the back tip of home plate appears to be the spot where the catcher interference calls go down to normal. Lacking robot umpires, I would institute a rule to keep catchers behind that distance to minimize the impact of both CI and pitch framing.