Thanks Tony. I already have a magnetic bubble level. In Longacre’s instructions they are sayings to use a total of 40 degrees rotation (20 to the right and then 20 to the left). Thus, it looks like the chart above is assuming 40 degrees rotation.
From Longacre:
To read CASTER first turn the wheels 20º to the right when setting the RF and to the left when setting LF. If you don't have a set of turn plates you can use the angles machined on the corners of the gauge as a reference. Turn the wheels until the angle is parallel to the side of the car. Rotate the gauge until it is level. Turn the knurled nob in the center until the center caster vial reads 0º. Now turn the wheels back past center to 20º the opposite way - for a total of 40º.