For Airsoft applications, since you aren't gonna be firing at something 500m away, ha ha, set the rifle down on something and look down the emergency sights on top of the carry handle and see what they're lined up with. Pick something a decent distance away...not too far, but not too close either. Then, adjust the optical sight so that the reticule is lined up on the same thing. Make adjustments as necessary. That's the easiest way to do it, and it works quite well with mine.
It's all fun and games until someone gets hurt...and then it becomes sport!