Actually, that will give you a rough estimate but a more precise method would be to drive a set distance, then divide that by how many miles or OD says. That will become your multiplier.
If you have a GPS or use Google maps and it says a set distance is 50 miles. But you drive that path and your OD says 48.5 miles. Then the math is,
Actual Distance / OD Distance = Multiplier
50 / 48.5 = 1.030927835051546 (1.03)
So now just multiply your OD or Speedometer reading by 1.03 to get a more accurate reading.