Since you love stats so much, let's look at some numbers...
You say that in 5 minute quarters, your RB will only get 8 carries a game. You also say that 6/7 minute quarters are perfect...
Well, 5 minute quarters are a 20 minute game...6 minute quarters are 24 minutes...and 7 minutes would be 28 minutes...
8 (carries)/20 (minutes) = .40 (ratio)
.40 (same ratio) * 24 (minutes) = 9.6 carries
.40 * 28 = 11.2 carries
Still doesn't look like you're getting your realism
So quit being so picky.
That was your math lesson for today kiddies, now take out your sex ed books...