There are different levels of correct when it comes to subs.
I find for music it's best to have it match the mains, but for movies I like to crank it until I can localize it. Movies sound better with good LFE. This is why we use such monster subs for movies.
There are many theories on sub levels, but the two most resonable groups are the house curve group and the flat line group.
to do either you will typically need an EQ. For this I suggest the Behringer DXC2496 for money folks, and the Behringer Feedback Destroy 1124DSP for poor folk.
A house curve is a rising spl curve from around 500 to 30 hz. The gradient depends on the room size, and listening preferences. Wayne at the Hometheatershack gives a great overview of house curves on their forum and it's a must read IMO.
The flat line group wants a flat response from 20khz to 20hz. Of course this is not realistic in most cases. So usually it ends up being 15khz to 30hz which is the range most of us can hear in anyway.
To get a house curve you need to start with a flat line and then adjust it. This relies totally on your ears and room. So no one has a magic formula.
Have fun.