You don't have to worry much about overfilling a 300D or 300SD if the car is level. You don't have an axle tube like the typical American vehicles. You have a differential box and then axle shafts. The differential box is mounted to the car and it doesn't move whether the wheels are supporting the weight of the vehicle or not.

So, no measurement is needed. Fill until the differential is full up to the fill hole and starts running out.

Ken300D