The advisory circular for experimental aircraft testing 80-99B, states to do flow testing at 5º above the highest anticipated climb angle. Understanding that Van's designed the RV12 for E-LSA, so therefore they've already done flow testing to ensure fuel flow at that highest anticipated angle.
I did my RV12 E-AB, so I did a full fuel flow test. I have found that my 12 can climb at 15º for a while, so 20º makes sense for testing.
The PAP fuel procedure is really just to help you calibrate your fuel gauge and mark your fuel tank for minimum fuel, not perform a full fuel flow test.
If you pull the RV12 tail to the ground, your nose angle will be 15º up. You can certainly put blocks under the main wheels to get to 20º if you desired, which is what I did. Just note that this testing was done at whatever angle and minimum fuel available at whatever angles you ended up with and keep that in the POH for future reference.
I'm sure the pundits will be along shortly to restate this more eloquently...