This tool helps to locate the position of the camshaft to ensure the brake pump pushrod lobe is on Bottom Dead Center.  According to the Factory Service Manual, the pushrod depth is checked at BDC and the measurement needs to be between .522" and .525".  A depth micrometer is necessary to check this measurement.
For your reference, the cam lobe that drives the pushrod cup is perfectly round.  It is not egg shaped like the valvetrain lobes.  That means there is no "base circle".  The cam has to be directly on Bottom Dead Center to take an accurate measurement.  A cam that is off by 5 degrees will result in a .001" error. 10 degrees will result in an almost .004" error.