RD is a very loose and obvious indicator of secondary success...throw out all runs where the game RD (either way) is greater than 10, and it becomes more accurate...but the ultimate would be to break it down game-by-game:
(games in which runs scored is greater than opponent)
vs.
(games in which runs scored is less than opponent)
This should be a very accurate representation of W-L record...in fact, I'd be surprised if it didn't match exactly...
(haha)