There should be no "regulation distance".
A properly regulated double rifle shoots both barrels parallel from the muzzles to infinity.
If scope sighted, the group center from the left barrel should be not more than an inch or so to the left of point-of-aim, and that from the right barrel no more than an inch to the right of POI
...regardless of group size enlarging to several inches or more with increasing distance.

Only an "unregulated" rifle (or poorly regulated rifle to be kind!) will cross at a specified distance from the muzzles.

This misunderstanding that a scoped double must shoot a one-hole group directly under the cross-hairs is persistent, but patently wrong!