Finding the diff ratio for any vehicle.... I think?

[SomethingRattling]

Secure wheel chocks as hand brake will need to be off.
Place vehicle in neutral.
Jack up rear wheel.
At this point I used a small mirror propped up with a stand fashioned out of a wire coat hanger so I could see the prop from the side of the car.
Mark the position of the propshaft, I used white touch-up paint.
Mark the position of the rear wheel with chalk.
Turn the rear wheel, counting the number of turns, until both marks (propshaft and rear wheel) align again.

Rear wheel turns = 6 then the ratio is 1:3.67
Rear wheel turns = 10 then the ratio is 1:3.27

Ratio = 2 x (Prop turns/Wheel turns)

e.g. On my C240 (202.086)

Ratio = 2 x (11/6)

Ratio = 2 x 1.8333333333333333333333333333333

Ratio = 3.6666666666666666666666666666667 or rounded to 3.67

Ratio = 1:3.67

I think this is right but if somebody sees a glaring mistake please tell me.

Thanks to Bolide for the idea.