Thank you for all the math! I am a Latin teacher, not a math guy! But, from your numbers, I can now extrapolate a general rule for myself: that in a perfect 33/33/33 model with parity between D & R, a 16 point advantage among Independents would only mean a 5 or 6 point move. So a sample of 20% Independents would move things only 2 or 3 points. So, with a +4 Dem sample (which well may not hold), a +2 lead for Mitt is pretty damned good!
You got it :o)
Now I know who to ask if I want to be all cool and smart by saying things in Latin!