I use a 4' level and a plumb bob with the help of my wife. Even with her lack of enthusiasm it goes pretty fast. Center it up on the wheel and mark 4 dots/tape on the slab. Measure/adjust/repeat

 

My tire shop charges $25 to just check the alignment and give me a read out. What is that, almost 10 alignment checks for 1 of these (and far more accurate)? Once you have it dialed in, how many times are you really going to need the tool? I could justify it if I lived in the boonies, but like some have said, an angle finder, tape measure and a friend will do the same.

Just my .02.