I think I see a serious problem with the coffee cup method.
My having the quad sitting on that flat surface, he generated a tremendous amount of propwash which of course directly bounced off the flat surface and back against the cup, causing the ripples.
If he had the quad sitting on a thin metal grate at least 3 feet off the ground, that allowed the the propwash to pass thru, then that would have been a much better method, IMHO.
Rather than using a coffee cup, there is a free app called VIBROMETER for the smartphone that will give you a numerical vibration readout that is infinitely simpler to compare readings with.
The most serious problem of course was that there was no creamer in the coffee. Black coffee?? YUK