With my "golf ball" plug technique it doesn't make much difference how full the tank is. Running the car until the tank was more empty wasn't very attractive because of the poor performance I was trying to diagnose. I did drain out a couple of gallons by disconnecting the fuel line, but it was going so slowly, being mostly WVO, that I thought it was too much trouble as I had only a bunch of 5-gallon containers, so I would have to monitor the draining closely to avoid overflowing them.