Actually you want to be careful with this test. The higher the output of your system, such as MSD, the more it can jump. Also, a WEAK spark that "seems to be" jumping in this test may simply be something similar to corona discharge. In other words you may be getting fooled.
If you are failing quite a few rotors, there is likely another reason.......such as something wrong in the distributor clearances, or bad plug wires, way too much plug gap, etc etc that is allowing spark voltage to run "high." Spark voltage is not like a 120V outlet. Spark voltage goes sky high when unloaded/ disconnected. So the "chain" of the output determines what that is. The coil wire, the rotor gap, the resistance/ condition of plug wires, whether plugs are resistor or internal gap, and the gap in the chamber, as well as engine load and AF ratio all determine spark voltage, as well as whatever the system puts out in power. Higher power---higher voltage.