Ernie - guess it depends on whats in the chamber. My 7-30 Waters is more than I want with any rifle scope. I am thinking 10 inches maybe a bit less with that one. I need someone to watch me shoot it and spot mark the safe distance for me. On the other hand my .221 I can easily use a rifle scope on and get away with the standard 3-4" no problem.
I think prone may tip your head forward a bit more which in turn would require more eye relief to get the same sight picture. It would also depend on what you needed to see to find your target......if you can get away with backing off to the point of not getting a full sight picture but can live with just spotting your target you gain alot. Again I would need someone to spot me to measure this and of course the situation would determine the actual amount I could live with.