I would have probably executed the situation differently. I think this is a classic example why you should be prepared to use your horn. You sensed that there was something strange about the behavior of that vehicle, so covering the horn would have been a better option. As soon as she started to swerve into your lane, a blaring horn probably would have put her back into her lane. Just my opinion based on your scenario that you just described.
The bottom line is that you were able to execute a good emergency stop. You can practice all you want, but it's not until you experience it in a real situation that you can reevaluate yourself on how well you were able to stop. Pat yourself on the back not only on your stop, but to spot the potential hazard ahead of time. Classic example of \"SEE\" Search, Evaluate, Execute.
Get use to using the horn, it's your best friend. Most riders forget to use their horn in many situations that could have prevented them from crashing. The horn is the easiest, and one of the best defensive tools available to a rider, it's a shame that most riders are not trained to use it instinctivley like braking. Practice using the horn and get a louder horn than the stock too.