The servo travel will depends on the servo you use, some of them will do more than 180, you can even get linear servos. There's no real standard. I doubt that one timing that gives 120 degrees on one servo will give exactly the same angle on a different one. It may be better to just use say 100 for full travel one way and -100 for full travel the other, or 0 to 200 to avoid negative numbers. To avoid excessive travel, you can do what the RC manufacturers do, which to limit the travel to maybe 80% of the availible. Just a thought.
-Z-