Brother Ovid,

Having followed along for a few years now as you've related stories about how flakey your clients can be, I respectfuly submit that you are trying to solve the wrong problem by turning this into an issue about your personal productivity. By looking for ways to justify your productivity when you're dealing with a client who can't or won't clearly articulate requirements you are making their problem your problem. That's a recipe for driving yourself nuts.

The best you can hope to do in the situation you're in is to satisfy their requests in an amount of time that's reasonable to them, at a level of quality that's acceptable to them, and for an amount of money that's reasonable to them. It sounds like you're doing this, but if it leaves you feeling defensive about your productivity, perhaps you need a stabler environment.

If you insist on measuring something, measure the stability of the requirements. Track requirements changes, and graph, over time, the average number of changes per requirement, and mean-time-until-change. This shifts the focus back where it belongs.