Originally Posted by Adam
I think you have answered your own questions. You need to either set it up so everything is square and learn to hold the rifle without canting (butt plate adjustment may help), or set it up so that when holding it the way that is most comfortable to you, the crosshair is vertical when the centre is plumb above the bore. Whichever way, you need to make sure you are consistent with it. One of the levels that fit on the scope tube might help here, as you can adjust it to your requirements.
Yes. You need to trust the level and not your eyes.
that's why we have levels because the brain is easily fooled and the you can't the rifle even though it looks and feels right. If you can't then your shots will move in that direction.
You say you need to add clicks, but at 10m even a 1/4 Moa click is a tiny amount. It is 1/40th of an inch, around 0.5mm. Can you see this difference?
I assume all this is being done indoors, correct?