Rifle selection (and shotgun, as well): Mount the rifle to your shoulder with your eyes closed and get a good cheek weld to the comb. The length of pull should be comfortable.
When you open your eyes, you should be looking through the sights with no need to move your head--whether iron sights or scope.
A scope should be mounted such that when all is righteous, the distance between your forehead and the back of the scope is three, maybe four inches.
You're from BATFE? Come right in! I use all your fine products!