Some additional thoughts and information.
During my building project there were two issues I wanted to resolve. One was to determine usable fuel and the other, more important was to verify the integrity and functionality of the fuel system and its ability run the motor at high power settings. AC 90-89B describes a test which covers all that. I assumed the reason for the nose-high attitude was to put the boost and engine pumps farther below the fuel servo or carb inlet and ensure the fuel system can still push fuel up-hill at the prescribed quantity and pressure.
I think the main focus is on system integrity and proper installation. We are not testing a fuel system we designed from scratch. We are making sure we put the darn thing together correctly according to the instructions. One loose fitting can cause your flow test to fail completely. Ask me how I know.
Your DAR may be impressed by your thorough testing if you do this test nose-low but I would expect them want to see it done per the AC as well. I may be mistaken on that.
My stock RV-14 fuel system works great nose-high (+15 deg) until the fuel level reaches about 4-6 ounces and then starts blowing lots of bubbles. Beyond that, I think the idea of unusable fuel becomes a little subjective as someone else pointed out. I’m confident my motor would keep running in a descent with around 1.5-2 gal in a tank but if that’s all I have onboard I’m looking for a cow pasture to land in
From AC 90-89B Section 11 - Additional Engine Tests
e. Fuel Flow. A fuel flow and unusable fuel check is a field test to ensure the
aircraft engine will get enough fuel to run properly, even if the aircraft is in a steep climb or stall attitude, and is accomplished by:
(1) Place the aircraft's nose at an angle 5 degrees above the highest anticipated climb angle. The easiest and safest way to do this with a conventional gear aircraft is to dig a hole and place the aircraft's tail in it. For a nose gear aircraft, build a ramp to raise the nose gear to the proper angle.
(2) Make sure the aircraft is tied-down and chocked. With minimum fuel in the tanks, disconnect the fuel line to the carburetor. The fuel flow with a gravity flow system should be 150 percent of the fuel consumption of the engine at full throttle. With a fuel system that is pressurized, the fuel flow should be at least 125 percent. When the fuel stops flowing, the remaining fuel is the “unusable fuel” quantity.
(There are more steps in the procedure. I just pasted the first two)
Anyway, hope this is useful information.