12 is about the 'regular' where you wanna tune it. as far as frequencies, the size of the port determines what the box is tuned too. typically, the longer the port, the lower it's tuned. i have mine setup for 33-34 hz for daily driving and i love it. most people i have seen around here tune low to mid 30's also.
there was a thread on amp tuning and they talked about slope, but i dont have time to search for the thread