This is how I've been doing it and it seems to work fairly well:
1. Level gun in gun vice with small bubble levels.
2. Place Black and Decker Bulls Eye Auto level on wall on opposite side of basement. I'm not sure how accurate the laser level is but I've checked it with a 4' level and it seems to be accurate.
3. Turn scope until horizontal reticles cover the laser lines.
4. Snug up scope and check for eye relief. If all is good tighten down scope and recheck. I add blue loctite to the screws individually once everything looks good.