I think you are going about this from the wrong direction...
Did the PO extend the oil pickup tube to fit the new pan ?
If yes,,,, then you can pretty much use the 8 qt figure....
The main thing is that you do not want to foam the oil by having the crankshaft or the air from the crank turning whip the oil.
I am assuming that the PO was trying to avoid this with the addition of the oil pan capacity... particularly since he put the extra space at the bottom as opposed to the sides, where many race pans are extended.
As long as you do not have the oil level any higher than it would have been when the engine was level and have 8 qts in the system you will be fine.
If you want to find out how high a CERTAIN amount of oil is, without drilling holes in the pan.... you can get one of those thermometers which stick to the side of propane cans to tell how much is still in them..... then pour already hot oil into the pan and watch the thermometer show the oil level. Keep marking your dipstick to calibrate it...
It sounds like you are trying to " take advantage" of the larger oil pan, when the PO may have only intended it to counteract the tilt of the engine to avoid foaming....