Unless you get into shimming and the like, the mount is going to be only as consistently parallel with the rail as the parts allow. The optic doesn't really care as long as there's enough az/el adjustment to satisfy your goal.

I just lock the AR in a vise and use a carpenter's level on the rail. And I only use the level to satisfy my ocd and not for anything pertaining to the accuracy of the optic / firearm. Mount it all. Use a thin string at a few yards with a weight hanging from a curtain rod (like a plumb bob) in the house. Align the vertical hair of crosshairs to the string. Carefully cinch it down, check the alignment, and call it good. It's all relative. Go zero and high-five myself for living the simple life.