It depends on the geometry of your polar mount if it is even able to go that far without flopping over. My 6 foot dish had an 18" actuator, and that worked just fine. My 10 foot dish has a 24" actuator, and that works fine for that one... I would suspect an 18" would work for you as well.
It's easy to measure it with a tape measure before-hand by manually moving your dish, and then check to see what the maximum extension is on a particular actuator.