There are a few different ways that you can determine the proper eye relief. The first is to simply mount the scope and look through it. If the image is clear and you can comfortably keep both eyes open, then the eye relief is likely sufficient. Another method is to mount the scope and then slowly move your head back and forth until you find the sweet spot. Once you have found the spot where the image is clearest, you can mark it and then mount the scope in that position.