I wish there was a simple way.... but there is none.
Once the bases and rings are on, I put the scope in the rings without tightening it. I then go into a standard shooting position (prone for me) and then slide the scope as far forward as possible and still have 100% scope picture with no shadow. I then tighten the rings so there is just a little bit of friction to keep the scope stedy, but still easy to rotate within the rings. I then use sandbags/bi-pod/pillows/ anything else I fan find to prop the rifle up, adjust it so it looks pretty dang level (yeah, guestimate). I then set back from the rifle, rotate scope/crosshairs, then standard position, then step back, etc etc etc. Looking, adjusting, as many times as it takes so it looks good. It helps if you have a known horizontal line to use for reticule alignment, but even that can be hard to find. Tighten rings when done.
This is just my Kludgy way of doing it... someone else probably has a better system.
Mel i used the same way... its pretty accurate but only as accurate as my eye (has its good days and bad days...) the real funny thing is... i have a scope.. but no rifle to mount it on so i do it off a table. thanx for the tip