As Neil pointed out, it is useless unless you have a gun that shoots perfectly 50/50, and dead center left to right.
I made plexiglass circles for 12, 20, 28, and 410 that represent a 90% pattern, once the pattern is shot, I position the circle so it covers the largest percentage of shot (takes a little wiggling back and forth and eyeballing). That lets me know the exact center of the pattern.
I never really got down to comparing different size rings, quadrants, ect, but my method is necessary as a starting point anyways, because unless you know the exact center of the pattern, everything else is useless.
Shotgun Insight works well too, but only for 12ga patterns...