Assuming your scope manufacturer certifies that the flat bottom area is parallel to the horizontal reticle line (and most do):
Just set the scope slightly loose in the rings, put a metal ruler leaned forward in a slot of the pic rail, and attempt to tilt it vertical, simultaneously contacting the flat bottom of the scope turret area with the upper surface of the ruler, and the flat surface of the pic rail with the bottom surface of the ruler. It will true the scope to the pic rail, then lock it down.
It's now mechanically squared to the pic rail.
Done. Go shoot.