I'm by no means an expert, but to me, it is fairly easy to see if it is perpendicular to the ground but harder to see if it is parallel to the wheel to wheel axis. There are two planes at work, and one is easier to see. I think I am explaining it poorly, but if your rear wheel is a clock face, easy to align parallel to the 12 to 6 line but much harder (for me at least) to get the 6 to 9. That's how the tool became invaluable to me.