Don't know if you have enough miles for an F award, but I just returned from using one on IAD-SFO-AKL-LAX-IAD. The FA told me that the SFO flights are usually pretty empty in F. She even mentioned some flights with only 1 person up there. The SFO flights are only a few days a week though. I didn't end up getting F on the US transcons, but did get C with no problems.