Setback is specified in degrees, and should generally be less than half a degree. If the left wheel is further back than the right, setback is negative. If the right wheel is further back than the left, setback is positive.
Setback is measured with both wheels straight ahead, and is used as a diagnostic angle along with caster to identify chassis misalignment or collision damage. The presence of setback can also cause differences in toe-out on turn angle readings side-to-side.