Your math looks right, but the closest satellite would be 72w. I am not sure if they are the same satellite, but AMC6 is at 72w. That is the one I would aim for, blind scan, tweak it to true south satellite and go from there. First, BYPASS the motor, and setup your dish for your true south satellite (AMC6), scan in that satellite so you have something to work with, then hook the motor up, and adjust it for your area. I just installed my motor Friday, and I lined up the motor/dish assy to south (had a compass about 3 feet in front of the pole, on the ground), Then I went into my receiver, entered my lat/long. I then motored to IA6 (my true south satellite), and then I moved/tweaked the motor mount/dish assy while watching the meter on the receiver. I then adjusted the elevation on the dish. Mine went up in minutes! I am still amazed! But i think you better go for the closest satellite to your true south. 72 would be closer than 79.