How do you ensure it's on the scope perfectly? — 3 Answers

With a quality 4' level mark a horizontle level line on an appropriate wall. While holding rifle as straight up as possible with scope attached align horizontle scope reticle with "level" line on wall. While maintaining horizontle reticle alignment with level line on wall adjust bubble level to read level. In other words horizontle scope reticle should align with level line on wall and scope bubble level should be adjusted to read level at same time. Then tighten screws on bubble level.

I might be thinking wrong, but I don't think so. The idea of setting up a "plumb" line and I assume at that point you hold your rifle with attached scope in such a manner as to align the verticle reticle line in the scope with the plumb line. At that point while maintaining plumb line alignment you hold your horizontle reticle "level" with plumb level. Two things: it would be about impossible to maintain verticle plumb line alignment to plumb string and at the same time "level" Horizonte reticle line in scope with bubble level. Simply put your "crosshairs" on scope are at 90° to each other, so....... if you "level" horizontle reticle using scope level your verticle scope reticle will automatically be "plumb", meaning the entire process of plumbing verticle reticle in scope using a string and plumb bob is totally 100% unnecessary. To my way of thinking you attach bubble level to rifle scope and hold rifle in such a way that bubble level reads level at time of shot and pull the trigger. No plumb string required. �