[Vitruvius]

It sounds like you may have to set something up as trial and error.
First, find your true south... I would use sun calculations for that.
Mount your dish to a true vertical pole and tune into your true south satellite. Once it is obtained, measure the angle set for your dish and this is the angle your dish will have to be when it is mounted on your motor.

Then install your mounting pole so it is not truely vertical. Install it so it leans to the north, say 20 or 30 degrees. What ever angle it leans to the north, set your motor to that angle and see if that gets you on an arc.... Mount your dish and motor to the leaning pole so it has the same angle as when you had the stationary dish setup. Your pole needs to lean to true north. Basically, by leaning the pole, you are tricking the motor of thinking it is at a different latitude. This will take a lot more adjustment than a proper setup.

The other option is to use an HH180 motor which seems to have a simple arc path that can be aimed more directily to the arc. I am not sure if that motor works with an offset dish though...

good luck