I find that if I try to get the reticle plumb by eye it never is actually plumb. No idea why but that is how it works out for me. When it looks right with the rifle on my shoulder it never is.

After finally realizing this, I use a bubble level on the rail and a plumb line to set the the reticle so they are aligned as close to perfect as I can get them.

If I then shoulder the rifle and look through it, virtually ALL the time the reticle appears canted when in fact it is not.

As for the scope rotating when you tighten the ring top screws, that is very common. You need to monitor the reticle alignment to the plumb line and check the bubble level o the rail constantly as you tighten the screws. It can be pretty frustrating sometimes but if you get it plumb to the rail once, then you never need to do that again unless you loosen the ring tops for some reason. You can move it from one rail to another and it will remain plumb.