What do you refer to when saying track miles? 6DME or 6NM (Calculated route FMC)
When you've got 6 nm left to fly. (you should be 1800 AGL). You can come in on a visual from any direction so the DME, there is an on-site DME, is not good for you anymore. Except if you're already aligned with the runway. The FMC distance will be whatever is programmed in the FMC. It's getting a bit academic now, a visual approach is primarily a visual maneuver and those who are experienced with it will develope good eye-balling. It will all make sense eventually, your first experience with visual maneuvering will be your base training. It's plenty of fun!