This problem absolute drove me nuts. I have tried just about everything and this is about the best I have come up with. My troubles were coming from when installing a level, and the level changing things sometimes true and others were jutst plain what the heck.
I took a piece of flat steel and milled it to fit in my bolt race way. I then level off of it. But before I mount the scope if I am putting a level on I install that to the scope based off of a level on my turret or cap.
With this all said and done I still do not trust it. I will then go to the range and verify that whoever assembled the scope actually put the reticle in true to the turret cap.
I can help with this but sure everyone is aware of what is next.
This also drove me crazy up until about five years ago. The last five LR rigs I have set up proved to be aligned very well out at long range as well as verified when tested with a plum line through the turret adjustment range. Once all is aligned using the standard rail, turret, levels, etc., I then with the rifle level and mounted securely, and my scope level mounted in final position, attach a Bushnell Collimator that contains a grid with MOA divisions and a range of +30 MOA. With the grid attached and aligned vertically with the scope reticle, I will then elevate my turrets 20-30 MOA. The vertical grid line of the scope reticle and collimator should be in perfect alignment with no angular deviation when run through the elevation range. If not, it's out of alignment, and all gets adjusted. This approach has worked superbly for me and I have yet have had to readjust my scope when range tested. It is also a good test for turret reproducibility as I had one well known high end scope indicate a turret problem which was ultimately verified and corrected by the manufacturer. I have also learned to trust my alignment, even when the reticle visually appears cocked to some degree. It's almost always my visual perception, not the rifle/scope. IMO.
"Let us speak courteously, deal fairly, and keep ourselves armed and ready"-T. Roosevelt
I have a bad habit of wanting to roll the rifle into my shoulder,,I think alot more people do this than will admit.As said above I've learned to trust what my bubble is telling me regardless of what the reticle looks like.
I read an article on here a while back. This will correct for many of the manufacturing deficiencies that can happen, assures proper alignment and will make long range shooting on the money.
Zero the rifle at 100 yards.
Make an inverted T target with a good square that has a vertical line about 20".
Take a plumb bob or level and make the vertical line vertical when placing on your backer.
Start dialing up and see it your hits are on the line or start to stray one way or the other. It will also verify if your scope is tracking perfect or not.
If they stray right rotate the scope right, left if it strays left. Very little is all that is needed.
Oh, after adjusting make sure you re-zero and repeat.
I shot with it, and at 100 meters, with a plumb line on the target, shooting 5 minutes of adjustment it tracked properly.
But something is still amiss.
Everyone who looks through the scope thinks it is way off.
I had a friend who is a machinist and has incredibly "accurate" eyes, (he can spot levels and give you measurements in the tenths of milimeters), look and shoot it and he thought it looked as bad as I did.