I agree with silhouette.
I have installed siding for a living, and I trust the degree of horizontal accuracy it provides when lining up a horizontal crosshair.
I line up the screws holding the butt pad on with a plumb line. This gets both rifle and crosshair on the square.
Once I do that, I mount the bubble on the scope. In my book, that's how I prefer to get first things first.
Greg