Ya'll are talking about two different things, (a) fuel flow and (b) unusable fuel.
In terms of fuel flow, Bret is good to go. He has 45 GPH after the regulator, while maintaining 35 PSI in the fuel rail. Tee a valve into the fuel rail and bleed off the equivalent of fuel burn at WOT full rich plus some for the FAA (say 17 x 125%, or 21.25 GPH), and the tank return will flow at 23.75
because rail pressure is regulated. The pump supplied the rail with 45 GPH either way.
Unusable fuel is not found at Vx, i.e. highest expected angle, nose up.
Here's what the FAR dictates for a certified airplane:
23.959(a)
The unusable fuel supply for each tank must be established as not less than that quantity at which the first evidence of malfunctioning occurs under the most adverse fuel feed condition occurring under each intended operation and flight maneuver involving that tank.
Most RV models place the pickup in the bottom rear of the tank. This SB provides an illustration:
http://vansaircraft.com/pdf/sb06-2-23.pdf
So, the most adverse fuel feed condition of all possible operations would be
nose down. How far down? Ahhh, let the debate begin! One camp will say pitch down so as to result in best glide, another as appropriate for a typical power off descent, and a third as appropriate for nose down at Vne, throttle against the idle stop. In all of these, the fuel pool in the
front of the tank is unusable, the only difference being how much.
Of course, we could argue the most adverse condition would be hard slip toward the tank in use. That would make about half the tank volume unusable