I use a Tipton gun vise (actually any vice will work with rubber jaws) . I first level the rifle using a round bubble level the top of the rail or any flat spot on the action. I then mount the scope & level it using the same round level. At that point your done. I confirm using a plum line.

https://www.amazon.com/Camco-25573-...mp;psc=1&refRID=QJCTCJM5ATHNRMH4B6M3

https://www.amazon.com/Accuracy-All...mp;psc=1&refRID=WSG2QRX76JKZTT7KE0QC