I think it all comes down to who is healthy and so far, I would bet that it's Lacy that lead the Seahawks in rushing.
If he's healthy and gets 15+ snaps per game, he'll get to the 700-850 mark.
I don't have faith and Rawls or Prosise can stay healthy enough to get the reps needed. I agree with your numbers and thinking that we can have a solid RBBC this year.
If they are all healthy, I see us making it to the NFC Championship game no doubt.
And in other words, no one worth taking in Fantasy unless it's your 3rd or Flex position.