II haven't found a thread for this so forgive me if I've overlooked it. I have tried countless level apps and plumb bobs but even when I place my phone against the wall, in order to get a true vertical or horizontal plane, I have to tilt my phone a few degrees to the left, which means I could use any arbitrary lines. Another way to describe it, is if I use a real level to orient my phone to a true vertical position, the lines are not.
