Its helpful to think about it the way power is applied. When power is applied to the differential by the engine the differential allows the rear wheels to turn at different speeds. When one wheel can not turn the other turns at twice the gear ratio.
So if your ratio is 4.11 that means the drive shaft turns normally 4.11 times for every time the axle turns, or 2.055 times if one wheel is stopped.
Here I think bigbad misspoke. If you stop one wheel and turn the other ten times, you should count 20.55 turns of the drive shaft. Now divide by half of ten, and you'll get the correct ratio, or 4.11
The more turns the easier it is to get an accurate number, but if you have looked up the probable options you rarely need more than a turn or to to narrow it down the the most likely ones.
