I don't think there is too much mystery to it: if you are flying at 100 kts into a 100 kt headwind, you could have the most amazing SAR but it would be meaningless as the SGR would be 0.
So SAR is only half the story. However, if you can predict SAR from the FM in advance and couple it with forecast winds the story changes.
You can't beat the actual SGR figure on the day though, and there is no simple formula for calculating it (as originaly asked).