My starting point
I use BKL mounts and I have a very level flat section of counter top. I sit the mount on the counter and put the scope in the mounts with my level on top of the turret. I set it level and mount my bubble to the tube, then mount as a unit to the gun. I have always been happy with how the reticule looks to my eye even though the vertical and horizontal never look at 90 degrees. I always want the horizontal part to look level, but this makes the vertical part lean to the left. My eyes make even the sight picture look oblong and not round. I just trust my level that's mounted to the scope tube and all seems to work. Frustrating to say the least.