As with most magnified ACOGs, the eye relief is about 2 1/2 inches, basically need to keep your face close. If learned to shoot with both eyes open, this becomes less of an issue. A few different mounts can be used, just do a simple search on this site.
While the advertised eye relief is 2.5”, it’s much much more than that in real life. I can say with 100% certainty that you can easily have a usable sight picture with you eyes 6-8” away. I run it at the mid-point on my upper and with the stock fully extended I have zero issues getting on target. This is a great product and I highly recommend it.