Bubble levels keep the sights elevation axis in the true vertical and the rifle's not canted. The amount a bullet will strike away from the aiming point horizontally is easy to calculate.

Multiply bullet drop in inches at target range by the sine of the cant angle.

For example, lets take a .321 Belted Magnum shooting 234 grain boattails out at 3210 fps. These bullets drop 300 inches at a thousand yards. If the rifle's canted just one degree, the sine of 1 degree is .1745. Multiplying 300 times .1745 we get 5.23 inches; five and a quarter inches in plain numbers. So the bullet will strike a bit over 5 inches to the right.

A .308 Winchester shooting a bullet that drops 450 inches at 1000, it would strike almost 8 inches to the side with a 1 degree cant.

There's virtually no change vertically as the cosine functions to show that end up with only a few inches difference for most situations. For the .321 Mag. example at 1000 yards above, a 10 degree cant will cause a 52 inch wide shot horizontally, but it would be only 5 inches lower.

Competitive rifle shooters have used bubble levels on their aperture front sights for years to ensure the rifle's not canted. There are some available for scopes, but they're hard to use as they stick up too far atop the tube. I'm not aware of any scope that has an internal one although such things have been suggested to scope makers.

Here's a picture of one on a .308 Win. match rifle and another showing what a bullseye target looks like through one. A small level's put on the rear sight's windage arm, leveled, then the muzzle band's loosened, turned to level the front sight bubble, then tightened. One can calibrate how much cant's needed to move impact one MOA at any range. I've done that in competition to make small windage corrections without coming out of position to turn the sight knobs.