There's no way to give an exact answer because it depends on the actual speed that the person in 2nd was travelling in the interim between his getting there and when the person in 1st finished.
But to put in really simple terms, assume they're all running 10-flat (because the math is easy). That means they were travelling at an average speed of 10mps. Which means that each meter in a 100m race takes a 10th of a second. So if Johnson was 0.13sec ahead of Lewis he was a bit more than a meter up. Since they were running faster than 10mps at the finish (Lewis took 0.88 for his last 10m), about a meter is correct.
As to whether or not that was an all-time record, for future reference, note that if you go to the Archive section of this site (button at top of this page) you'll find all-time Olympic medalists, and they're arranged by event, so you could scan all the 100m differentials in an instant. You'd see that Bob Hayes '64 (0.19) and Carl Lewis '84 (0.20) both had bigger margins than did BJ.