It's a good question and I've wondered myself. Say, outdoors no wind and everything else being equal how does it drop off with distance? Proportional? Square of the distance? Step function? I'm pretty sure of a noticeable draft boost even 5 or 6 bike lengths back, and I'm guessing that if you're close enough it doesn't make a whole lot of difference from an inch to a foot. I could be wrong. Surely someone at some time has put together a decent approximation.