When the shooting eye is too far from—or too close to—the scope’s objective lens, the field-of-view (FOV) is reduced due to improper eye relief. We normally see this as a vignette effect or dark ring around the center of the reticle. Set eye relief while you are in the shooting position you are most likely to use, because the distance from the scope’s objective to your eye changes based on the position of your arms and shoulders. If, like most of us, you do not use one static position all the time, find a happy medium with your most common shooting positions.

Move the scope closer to the eye until the dark ring begins to appear while ensuring your head is where it naturally lies on the stock. Then, move the scope away from your eye until the effect goes away completely and you have a clear FOV. Continue moving the scope away until the vignette effect returns. Now reverse the process and find the midpoint where your FOV is clearest. This is the correct position for proper eye relief.

You should end up with an inch or so of leeway on either side of this midway point to help when shooting from multiple positions or from different magnification settings, which often change the optimal eye-relief distance on many scopes.

A simple handy guide from: shootingillustrated.com