The way you're using seems unlikely to work properly.  I assume your measuring panAngle every frame or on every mouse update.  Doing it your way panAngle would immediately skip from 360.1 (or it gets to the first time it exceeds 360) to 720.1 so you'll lose an entire revolution.  A better method is to have a separate variable that counts revolutions and add to this when you detect panAngle changing from just below 360 to just about 0 (you'll need another new variable recording the previous state of panAngle to do this).  Then you can do: totalAngle = panAngle + (revolutions * 360).

Jonny