Variations in eye relief will be determined by recoil,scope magnification(variable scopes) and even the clothing you wear,(tee shirt compared to heavy hunting jacket.)
You should set the eye relief at the highest magnification of your scope and remember that what is a safe eye relief for a standing shot may be inadequate at the bench and cut you badly in a prone position.
When you raise your rifle there should be no need to"adjust your head", it should be there and clear.

Eye Relief From Zeiss website

This describes the distance between your shooting eye and eyepiece lens. It's an important safety consideration. Because if the eye relief is too short, there's an increased risk of dangerous contact between you and your scope under recoil.

Eye relief is determined by the field-of-view, and by the focal lengths of the objective lens and eyepiece lens. Generally, the higher the magnification and the larger the field-of-view, the shorter the eye relief.

All Zeiss scopes, whether fixed or variable power, have consistent eye relief throughout their power range.

Thanks for the tip hammer1. That is especially important with this hard kicking muel I'm shooting! But my problem is that the scope is too far away and I can't get a full immage.

Quote:You should set the eye relief at the highest magnification of your scope and remember that what is a safe eye relief for a standing shot may be inadequate at the bench and cut you badly in a prone position.

I'd have to say that personally, I disagree with the first part of that statement. To me, eye relief is most critical when a hunter has an opportunity for a quick, close-in shot at game. At such times (actually most all times) a variable power scope should be turned down to the lower range of power, anticipating a possible close-in shot. Eye relief is most important when mounting a gun, naturally. I normally feel that a longer shot, perhaps needing added magnification, should give a hunter/shooter more time to adjust his head (cheek weld) as necessary to achieve proper eye relief at that power.

Note that this should not make any difference in actual eye relief available, but merely how long it might take to attain a full, clear picture. I own a few Zeiss scopes and actually Zeiss scopes also have some of the most  consistent eye reliefs in the business (meaning their scopes change very little from low to high power) Many quality scopes may change an inch or more from low to high powers.

I absolutely totally agree with the idea of using care with eye relief while at the bench. With many of my higher recoiling rifles, I may not even use the highest power, while shooting at the bench. This to hopefully prevent a case (another case) of "scope eye".  

As far as the question of "what" eye relief for the OP's .308 rifle, it's been my experience that any scope with a minimum of 3.5" of eye relief should work well for even a lightweight .308. I normally look for scopes with at least 3.75" of eye relief at some power, so as to give some latitude for change-out, if ever done. For heavy recoiling rifles, I like 4+" available. Simply my personal preferences, mind you.

It's been my expirience that despite the manufacturer's stated eye relief, the eye relief needed will vary depending upon the individual.

Have your scope mounted with you in the shop, testing it in various positions. If a shop cannot accomodate you, find another shop.

Biker