[inquisitor]

Why don't you just observe the signal strength while very slowly turning the antenna? Out of my experience I can tell you that - as long as you don't have a clear fresnel zone between your antenna and the tower - the ideal aligment is not necessarily the direction of where the basestation is located. Especially if there are obstacles in the line of sight you will often get a stronger signal from reflections coming from the sides.
A friend of mine has pointed his Yagi antenna to the opposite location of the basestation as the adjacent house reflects the signal and so provides much higher bandwidth than when pointed directly to the basestation.
For best results you should constantly transfer data (e.g. some streaming media) while turning the antenna and also watch how speed changes besides signal strength.