Final Cut- How to start audio fade from higher than 0db?
You need to use keyframes.  It's been a few versions since I used Final Cut, but they are generally represented by a little diamond indicator.  If you add a keyframe at the point you want the level to start increasing and another at the point where you want it to stop increasing, you can then adjust the value at the later point and the volume adjustment should automatically be performed over the time in-between the two keyframes.

More information on audio keyframes in FCPX is available from Apple here.