The popular voter differential is 1.9%
No, the popular vote difference is 1.9% (50.2 minus 48.3).
The raw differential is half of that: 0.95% - if 0.95% of the vote goes the other way, Obama and McCain are tied.
Given that the difference given by Battleground in the past has varied by as much as 0.45%, 0.86% is where the differential would be on average.
But we can use 0.95% if you prefer.