Correct...one wrench means service schedule "a" I believe.
The FSS counts down miles until it's time for an oil change...It displays the FSS miles when the car starts, or when you push the little button next to it twice quickly.

One thing to note...the FSS miles are not "real" miles...it's not impossible to drive 500 miles and only see a 300 mile change in the FSS miles. It all has to do with the type of driving conditions, and the type of driver!