So I eyeballed the scope and hope I got it straight. One way is to assume the top of the rifle is square. Find a square and level surface on the bottom of the scope and rotate a shaft of metal between the two parallel surfaces of the top to the rifle and the bottom of the scope.
Many things have to be aligned in several perfectly inside the scope (dimensions and planes), on the rifle, rifle to the scope, and the holding of the rifle vertically when fixing the scope to the rifle. If not, small angles get magnified as you go to longer distances.
Just putting a bubble level on the top of the rifle and then the scope doesn't seem all that reliable, but maybe it is? I wonder how precise/accurate those bubble levels are?
Then one has to hold position the rifle correctly when aiming.