Rule of thumb....set it to what sounds best. Ask any installer and 9/10 times they set it by ear. Unless you are competing, then they only thing that matters is how it sounds to you.
Will the amp your using take a high input voltage??? You may need to use a LOC (Line output converter) to step-down the voltage so you will not damage your amp...
Speaker level outputs is much higher voltage than line level (rca). To do this you will want to purchase a LOC - Line Output Converter. It has speaker level inputs, and line level outputs.
I use the ADP 170's in my system. Due to less-than ideal room setup they are mounted at 7feet above the ground, and they sound very nice. They came with heavy duty drywall plugs that have held the speakers in place for over a year (mounted directly to the drywall, not studs)