MIKE GEORGE says:
There are lots of variables here, the recoil of the rifle and the magnification of the scope being two of them. With the scope off the rifle, I’d start by holding it up in front of my eye and seeing how far away I could get it and still maintain the full field of view.
This is likely to be more than three and a half inches, and should give you a safe starting point.
Generally speaking, the higher the magnification of the scope, the nearer it needs to be to your eye in order to get the full view.
Remember, it’s better to start with the scope’s eyepiece too far away than too close. Scope bite from a powerful rifle doesn’t just hurt – it can cut you, and smash your spectacles if you wear them.
I suggest you then try a few shots, remembering that in the field the terrain may force you to adopt shooting positions which are far from ideal, as well as the usual ones of standing, kneeling, sitting, prone, and using a stick to steady your rifle. Some positions may put the scope uncomfortably close to your eye if you opt for the minimum eye relief.
About 30 years ago a member of the British Sporting Rifle Club gave me a lesson with a stalking rifle on the Bisley ranges.
His advice was to always try to shoot in a comfortable, head-up position: “The image in the scope should be like sitting in your armchair at home, watching TV”.