I have the BX1500D, and my gain is about 1/3 of the way up. 3/4 of the way up is if you have a low preamp voltage coming from your HU. I have approximately 4volts coming from my HU.
Still, the best thing to do, im my opinion, is similair to what the last guy said. Turn your gain all the way down, turn your HU to the max level that you listen to it, or until your in-car speakers start to distort, which ever comes first, and then gradually turn up the gain until you hear distortion, then turn it back JUST A HAIR.