Re: scope /eye releaf
Your question is pretty vague, but I think I know what you're asking. The eye relief (ER) of a scope usually depends on the quality of the lens system. That being said, most all scopes have 3-3.5" of ER. Like the last guy said, ER of one scope can vary depending on power settings (i.e., at 6.5X ER is 4.5", at 20X ER is 3.0"). Just the nature of the beast.
The way I mount a scope, with regards to ER, is after bases and rings are installed and aligned, I then take the top half of the rings off. There are different types of rings so try to mentally picture this. My rifle is usually setting in some type of rest. I lay the scope in the bottom half of the rings and ensure it doesn't touch the barrel and has necessary clearance. I've usually checked out the ER beforehand so I have an idea of what it's like. I lay the scope in the rings, install the top half but don't snug down the screws, close my eyes, gently bring it to my face and see what the sight picture looks like. From here I know whether the scope needs to go forward or backward in the rings or if it needs to be tilted left or right. ER should be "natural" when the rifle comes up. If not you'll either be stretching your neck or backing off to see.
With regards to leveling the crosshairs; I make sure table, rest and rifle are level. I use a sticky note and put it across the house on another wall. I take my torpedo level and draw a level line (horizontally) on the sticky note and align my crosshairs up with that level line. Very simple, but works. Sometimes, if the room is long enough, I put a red dot on the sticky note and bore sight the rifle/scope. I have put the red dot on the back privacy fence and pointed the rifle through my sliding glass doors. Hope this helps. JohnnyK.