I do the same but rotate the tire exactly two full rotations and do the math, on the pinion yoke, driveshaft, if it rotates 2 and a little more than 1/2 turn it has 2.5+ ratio, if it rotates 3 and 1/4 it has 3.25 ratio and so on .
Cars with posi or spools you need to be able to have both tires spin and spin them one full rotation and count the rotation on the yoke or driveshaft , same deal 4 + 1/10 turn is 4.10 or 4.11, 4 and a little more than 1/2 turn are 4.55 or 4.56.
IHTHs