Unusable Fuel Definition
I certainly don't know that this language applies to Amateur Built aircraft but it sticks in my mind that unusable fuel is supposed to apply to the "most adverse" planned flight conditions, and that would seem to make sense.
If someone really wanted to determine this, shouldn't they, at the minimum, find out when fuel no longer feeds at the maximum normal nose up and nose down condition? Maybe not necessary to determine unusable fuel for a roll or reduced G maneuver, or maybe so, if that is normal.
At any rate, I certainly didn't go to those lengths, but to really do it right it would seem logical.
I have done the simple thing of running a tank dry in level flight, but that only tells me about that flight condition.