My experience with cell phone compass apps is that they are accurate to +/- 180°
Seriously, I find them entirely unreliable.
Aiming the HBU-33 is not going to require extreme precision. Simply point it in the general direction of NW. Then try adjusting in small steps, both left and right. Lock on the position that gives you the best overall reception reliability. You will find this easier if there is a signal quality meter function on your TV and you have a helper. Rescan for additional channels after adjusting the aim.