Right as far as it goes, read onswards
NO, no NO!!! Here's how you "do" an open rear end, and it's easy!!!
You DO leave one wheel on the ground!!!
And you go around and turn the jacked up wheel TWO revolutions while counting driveshaft turns. That will directly give you the ratio, IE 3 1/2 "plus a bit" is 3.55, etc
If it's limited slip (suregrip) then you jack up both wheels, and turn either wheel ONE turn, counting the shaft