Managing green with KPIs

Either by a lack of self belief, or by lack of belief in the ability of numbers to sum up a person's contribution, Ive never been a fan of numerical key performance indicators. But People love numbers. How old are you? How many square feet does your house have? etc.
Now Seth Godin makes a compelling case for the value of having a number attached to a green product. Interesting. Carbon footprint = distance from source to source x tranportation medium could be one example.
Miles from home could be a simpler, but less accurate measure.
Well, here's Seth's compelling post.