The results are in! At least after two of my tests. Both tests achieved an average rainfall rate of about 4 inches an hour using 500ml in a water bottle with a pin hole at the bottom. So, we know that 500ml is supposed to equal 0.92 inches of rain in the VP2. Here are the actual results
Test 1: 500ml = 0.89 inches
Test 2: 500ml = 0.90 inches
Some water may have been left in the tipper from test 1 to test 2, that could account for the extra hundredth. Regardless, these numbers are technically within spec according to Davis. I will also say that I performed this test during the middle of the day with bright sunshine, temp of 87, and dew point of 62. Some evaporation may have occurred, but probably not enough to make a real different.
Now, the question is, do I even bother adjusting the screws? My calibration tells me I'm low by about 2% on my gauge.
*Edit*, just opened unit to see the tipper. It was about half full at the end of the 2nd test. I think I was probably spot on at about 0.895 inches per test. 0.895/0.919 = 2.61%