The best way is to get a few props and try them. The first goal is to let the engine operate in the optimum power range. The second is to provide thrust in a way that works with the model. There's a number of diameter to pitch combos which will allow the engine to work in the ideal RPM range. The trick is then matching one of those options to the plane's weight and preferred flying speed. There's no doubt that since it would appear that your Ultimate is a bit overweight if the previous posters are right that your model will do best with a different prop than the same model which is at the optimum lighter weight. So the only decent way to find out is to try some different options. The props that don't work out? Either keep them for some future model that uses the same engine or sell 'em off at a club swap meet somewhere along the way.