I love crunching the numbers too. Ive fiddled around with formulas for RB too, but never have time to put too much int it. I expect it would go something like (avg.YPC multiplied by TDs, plus total yards per scrimmage, divided by fumbles?).
it would be better if there were a way to factor in the quality of blocking. There are stats available for the NFL by football outsiders, not sure what they go on though, nothing for college that I am aware of. Theres yards after contact that apparently get tracked for some affletic backs like Mark Ingram, no idea where they came from but it was just mentioned during a big game so during the Heisman debate with Gerhart and they kept talking about Ingram's yards after contact.
although many variables to measure the backs effectiveness in the type of system they are in. For instance, Gaffney gets a lot of run plays that are designed to only get a small amount of yards to eat up the clock, so some games his ypc drops towards the end of the game. Also, these type of plays offer less opportunity to get a big run, which was a knock of Gerhart coming out, he didn't have as many 20+ yard runs as Ingram or some crap, but as you can see in the NFL he can break those off with limited touches in a different system. I believe he did have the most amount of 10+ yard rushes his senior year.
Burkhead was used in a similar way his junior year and didn't have many big runs, but was very effective, his senior year with the knee injury the times he was in, most games with a knee brace, he had big runs almost every game used in a different way sometimes. He had quite a few 20+ yard plays in the Georgia game alone when the brace was finally off, and runs of 57, 61, 73 in 3 different games with only 98 carries that year which was a bigger than what he had the previous season with 284 carries.
people want to say it was his speed that affected it with his 40 time but as we can see in the NFL 40 time has little to do with it, Alfred Morris has the lead for most 20+ yard runs this year (9), and Arian Foster was ranked 4th in 2010, both run in the 4.7 range, while Jamaal Charles who is probably the fastest when you factor in 40 time and 100 meter time, has only 3 20+ yard runs this year.