I find that scope positioning forward or back is going to depend on multiple factors including the rifle and stock, the scope, and how you plan to use the rifle.
In my hands, if shooting from rest with the rifle supported on bags or a tripod, the eyepiece can be quite a bit further back than if the rifle is going to be shot freehand or from multiple positions (standing, kneeling, sitting, prone). The prone position especially tends to require the shooter's eye to be quite a bit further forward than shooting standing or kneeling or from a bench. And when prone, most people find it difficult to move the head further forward or back while keeping the butt properly mounted in the shoulder pocket. If you find you can easily move your head toward or away from the muzzle in a prone position, at least one of your parents was probably a turtle, so I wouldn't brag about it too much.
Properly used, the term "eye relief" is how far you can have your dominant eye positioned from (behind) the eyepiece and still have a full field of view through the scope without vignetting. Vignetting is the dark rings, arcs, or complete blackout that occurs when you do not have your eye properly positioned relative to the eyepiece. Eye relief is going to be different with different scopes, and with variable powered scopes, eye relief will decrease as magnification is increased.
If you get your eye too close to the eye piece you will also get an incomplete field of vision. The distance forward and back that you can position your eye with any given scope and magnification and still have a complete field of vision is called the "threshold" and this typically shortens up with an increase in magnification. It is also necessary to position the eye centrally with regard to the axis of the scope tube to get a complete field of vision. The "eye box" (which would probably better be called the "eye cylinder") is the cylindrical space within which your eye must be placed to get a complete field of vision through the scope. The diameter of this cylinder is the "exit pupil" and the exit pupil is the diameter of the objective lens divided by the magnification power. Therefore, scope with larger objective diameters have wider exit pupils, and for any given objective diameter, exit pupil diminishes with increasing magnification making precise eye positioning less forgiving of error.
Consider also that for unsupported shooting you will want to achieve a good cheek weld on the comb of the stock. The stock configuration can therefore partially determine proper scope position. Most scopes will result in a rise in the sight line relative to the bore line compared with iron sights, so if you are adding a scope to a rifle that has never had one, you may need to build up the stock comb in some fashion. The larger the objective lens, the higher the comb will need to be to achieve a good cheek weld.
If you plan to shoot from multiple positions, I would get into the position which places your head furthest forward with the least ability to adjust eye position forward or back. For most this would be prone. For a few it might be sitting. Standing and kneeling usually allow more latitude in head positioning. Once you get in this position, put the scope on the highest magnification setting. This will narrow the eye box and shorten eye relief to their minimums for that particular scope. Now move the scope forward and back until you find the position at which your eye is farthest from the eye piece while still maintaining a full field of view. With the scope in this position, as you decrease magnification, eye relief and exit pupil will both increase enlarging the exit pupil and allowing easier adjustment of dominant eye position.
In my hands, if shooting from rest with the rifle supported on bags or a tripod, the eyepiece can be quite a bit further back than if the rifle is going to be shot freehand or from multiple positions (standing, kneeling, sitting, prone). The prone position especially tends to require the shooter's eye to be quite a bit further forward than shooting standing or kneeling or from a bench. And when prone, most people find it difficult to move the head further forward or back while keeping the butt properly mounted in the shoulder pocket. If you find you can easily move your head toward or away from the muzzle in a prone position, at least one of your parents was probably a turtle, so I wouldn't brag about it too much.
Properly used, the term "eye relief" is how far you can have your dominant eye positioned from (behind) the eyepiece and still have a full field of view through the scope without vignetting. Vignetting is the dark rings, arcs, or complete blackout that occurs when you do not have your eye properly positioned relative to the eyepiece. Eye relief is going to be different with different scopes, and with variable powered scopes, eye relief will decrease as magnification is increased.
If you get your eye too close to the eye piece you will also get an incomplete field of vision. The distance forward and back that you can position your eye with any given scope and magnification and still have a complete field of vision is called the "threshold" and this typically shortens up with an increase in magnification. It is also necessary to position the eye centrally with regard to the axis of the scope tube to get a complete field of vision. The "eye box" (which would probably better be called the "eye cylinder") is the cylindrical space within which your eye must be placed to get a complete field of vision through the scope. The diameter of this cylinder is the "exit pupil" and the exit pupil is the diameter of the objective lens divided by the magnification power. Therefore, scope with larger objective diameters have wider exit pupils, and for any given objective diameter, exit pupil diminishes with increasing magnification making precise eye positioning less forgiving of error.
Consider also that for unsupported shooting you will want to achieve a good cheek weld on the comb of the stock. The stock configuration can therefore partially determine proper scope position. Most scopes will result in a rise in the sight line relative to the bore line compared with iron sights, so if you are adding a scope to a rifle that has never had one, you may need to build up the stock comb in some fashion. The larger the objective lens, the higher the comb will need to be to achieve a good cheek weld.
If you plan to shoot from multiple positions, I would get into the position which places your head furthest forward with the least ability to adjust eye position forward or back. For most this would be prone. For a few it might be sitting. Standing and kneeling usually allow more latitude in head positioning. Once you get in this position, put the scope on the highest magnification setting. This will narrow the eye box and shorten eye relief to their minimums for that particular scope. Now move the scope forward and back until you find the position at which your eye is farthest from the eye piece while still maintaining a full field of view. With the scope in this position, as you decrease magnification, eye relief and exit pupil will both increase enlarging the exit pupil and allowing easier adjustment of dominant eye position.