Metrics are a tricky SOB.
There are a few different metrics that you can track, some of which are more concrete and objective, and some of which are softer and subjective. In a situation where you have a captive audience, you're probably better off tracking some of the more concrete metrics, as the subjective metrics may have less of an impact on your audience than otherwise. Your primary goal is likely going to be making the workflow as quick and easy as possible. Different metrics you might attempt to measure include:
1.) Task success rate
2.) Back button usage
3.) Use of search or navigation
4.) Error Rate
The whole business of choosing specific metrics to track, what analytics you can use to track them, and interpreting those analytics to come up with meaningful metric meanings, is a topic you could (and many people have) write a book about.