When you say "trapezoid," are you referring to the fact that the front and rear track widths are different? How do you know how much they differ? I can see how this would all work if you were starting from a known reference (i.e. a calibrated alignment rack) but otherwise, you're kind of guessing.
With strings, assuming the strings are relatively close to the tire / wheel (the tire being the limiter, obviously) the angle of the scale when you take a measurement doesn't really affect it much. I don't even use a level - it's similar to the technique one uses when measuring a cylinder with a dial bore gauge - you look for the smallest readout on the dial. Same thing with the scale - you move it up and down until you see the smallest reading. That, by definition, is level. This of course assumes that the strings are level, but again, it's really very easy to verify that during initial setup.
This discussion is interesting to me because I make so many small alignment changes to the competition car(s) and commercial alignment setups are frequently so far off (or they shit the bed when a vehicle has excessive camber, as mine does). I literally check my alignment before and after every event, so being able to do that quickly and very accurately is essential.
You should stop by some time and I'll walk you through my method to show how easy it is. The bar is well-stocked
