How many disc revolutions make 1kWh on this kWh-meter?
The Kh is (indeed) what you're after, and the formula you give seems to be correct, according to wikipedia, 'Electromechanical' section.

P = (3600 * Kh) / t &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(for one rotation)

If we call the amount of rotations n, we get (P / n) = (3600 * Kh) / t

Or: P = (3600 * Kh) / (t * n)

Where:

    t = Time in seconds
    P = Power in Watts


That means the unit of Kh is:


(W / n) = (3600 * Kh) / s
(3600 * Kh) = W / (n * s)
Kh = W / (n * s * 3600)
And because s * 3600 = 1 hour: Kh = W / (h * n)


So if Kh = 7.2, and it takes 1 hour for the dial to rotate 1 time, you've used 7.2 Watts.

You want (n * kWh). Let's change that into (n * Wh) for now.


Divide everything on the right side through W: Kh = 1 / (Wh * n)
(n * Wh) = 1 / Kh


Now, we need to change Wh to kWh, so we multiply both sides with 1000:


(n * kWh) = 1000 / Kh


So for Kh = 7.2 you get 138.89 n*kWh or 138.89 rotations per kWh.

Your calculation is correct. :-)