Have you already tried this sort of proceedure ? I find it helps to take quick looks so your eye does not correct the focus .
Step 1Locate the reticle focus ring on the eyepiece. Unscrew the stopper, a small grooved ring that goes around the scope, enough to make the necessary focus adjustments to accommodate individual eye requirements. The stopper will remain on the scope during adjustment.
Step 2Point the scope up into a clear blue sky. Hold it far enough from your eye until the entire view through the scope has sky in the background. This will set the proper eye relief distance and eliminate any dark spots on the edges of the view that can prevent a proper focus. Pointing to a clear blue sky will also eliminate any background distractions and give a true focus based on your individual vision.
Step 3Adjust the focus ring until the reticle is completely focused. Find the range that is most comfortable with your eye and then make small adjustments until you reach a clear focus. When you find the focal point, the image will be the most distinct and clear.
Step 4Tighten the stopper to set the focus into position. This will help prevent any accidental adjustments to the focus that may happen if you bump the focus ring with your hand, gear or a brush.