Clear as mud?[/QUOTE]
I love this topic, because it's been on the Top 10 Most Often Gotten Wrong Questions on the USCG Masters Rules of the Road Quiz.
You're absolutely right: a 'steaming' or 'masthead' light is exactly as you describe - a white light at, or near the top of a boat's highest point, as close as practical to the vessel's centerline, showing an unbroken arc at 225 degrees from the boat's bow.
Why 225 degrees? Because the white Stern Light (135 degrees) makes up the remainder of the 'circle' - which according to my calculator equals 360 degrees. Hence, you use your 'steaming light' in tandem with your 'stern light.'
But - wait - there's more....
If you operate a motor vessel (or sailing auxiliary with engine ON and ENGAGED), and you happen to be 12 meters or less in LOA, guess what? You can use your so-called ANCHOR LIGHT (aka: All Around White Light) INSTEAD of the steaming light, so long as you remember to turn your STERN LIGHT off too.
Clear as a Seattle sky in winter? Methinx 'knot.'
Capt Dan G>E35II "Kunu"