well calibrating can be hard if you don't own measurement equipment, but with a little patience it can be quite good anyway...
start with seating your subwoofer in your listeningposition, then you start walking around in front of your listeningposition and try to find a position where you like it....that's more or less where you want to place your woofer...though a corner positioned subwoofer will ha significant lower distortion than a placement in the middle of the room...
when you settled where to put your sub you need to level it with your other speakers....put the volume for the sub at 0...and slowly start cranking it up until your bass takes "over" the music.....then you start lowering the volyme of the sub until you can't identify that it's really playing...
closer than that without measurment is impossible....
if you have tops and nulls in your room you should take a look at some acoustics either on the dedicated home theatre part of this forum or at my website (signature)
