Venom,
I like the
EXD level best. It uses an end V that centers it on the barrel contour, then a second sliding V that centers the other end over the scope bell. This puts the tool's centerline coincident with both the centerline of the barrel contour and the scope tube at the same time. The single level vial then tells you when you have the gun and scope truly upright in your cradle or gun vice. When you know it's truly level you can turn the scope to align the crosshairs with the fuzzy shadow of the upper V slot of the device or just focus on a plumb line to get a comparison for the crosshairs. I have a laser level that projects self-leveling vertical and horizontal lines on a wall, so I use that. Works a treat, and you can keep checking the crosshair match to the plumb line or laser line as you start torquing the screws to be sure you don't get a shift.