Do you need to be re-measured, Yes, and no! It dpends who did the shot gun fiiting. A maker of double rifles such as Holland's, if they did your shot gun fitting should be able to recalculate the measurements.

If you read Walsh, aka "Stonehenge" you will see that he covers this stating that your double rifle should have both more bend (or "drop") and a shorter stock. The cast should remain the same. I think?

I once attended a lecture by the author of a book on double rifles who came out with an absolute load of nonsense. That Walsh must have been very short and with a abnormal neck as he had measured one of his double rifles and deduced his build as if the measurements were the same as for a shot gun!

A shot gun is normally set to put 60% of the charge above the mark. A double rifle will be set to put the bullet on the mark. Further it will have a shorter stock as the pull length for a rifle will not be the same as for a shot gun.

I can't recall why, but it is to do with position of the head, I think. It's why what would be a short length of pull on a shot gun is considered and marked as LONG on the interchangeable stocks on a No4.

I believe that barrel time also is a factor.