Cheers fella ;-)specifically..https://www.youtube.com/watch?v=Sv1ChXJrH6c
Good callShine a bright light on the tube you're trying to decal, and you'll get a shiny reflective line along the axis of the tube. It'll never be crooked, always parallel to the tube axis. Let this be your guiding light- either for helpful tape-markers, or for the brave 'get-straight-to-it' application.