I used the AVIA HT setup DVD and a Radio Shack SPL meter to calibrate the level. It turned out to be much lower than I would have assumed.
Without that, you could set your speakers to small with an appropriate crossover frequency on the receiver, and play some music. Start with the level on the sub at nothing and raise it bit by bit until it no longer sounds natural, than go back. This level will depend on sub placement. Try near a wall or corner, or right behind your seat. When you have the level okay with music, then try a movie with a good LFE track and watch out!