Don't make it so complicated. Turn all the processing off or set to "flat" on your HU and/or EQ. It's basically just like doing it with a DMM. Volume at 3/4.
Disconnect the sub and hook the probe to the positive and the little ground to the negative and adjust the X and Y axes (left/right and up/down on the "D" pad) until you see your wave. Adjust the gain unitl you get the wave you want.
What size amp are you trying to set?