The "bore sight on a plumb-bob line" has done it for me for decades.
Clamp the rifle with it sighted on the line & the vertical cross-hair running down it and then bore sight that the plumb line is centered in the bore.
But then 300 yards is way out there for me so microscopic errors don't matter.
Anything you try on the outside of the scope housing (like milling levels on the scope caps) assumes the reticle is perfectly aligned with the housing. Maybe is, maybe is not.
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