that seems like extra math. Tim, your method might be important if we needed an exact number, but there are only so many ratios for the Jeep axles and so if it's about 3, you have 3.07. if it's about 3 and a half, you have 3.55. If there were infinite possibilities, then yes, spinning them more and dividing sounds like it'd be a bit easier to get accurate.
I just jack up one tire and spin it twice. (unless it's a posi, and then you have to rotate both tires once). It's super easy if you have a helper to spin the tire.