Rahul Vohra of Superhuman has a great post on First Round's excellent Review platform: How Superhuman Built an Engine to Find Product/Market Fit.
It's about operationalizing the product-market fit construct using Sean Ellis' (he of LogMeIn, Dropbox, and Eventbrite fame) customer development survey. The approach centers on the question: how would you feel if you could no longer use the product? And digging down on the further responses of "Very Disappointed" and "Somewhat Disappointed" users.
Sean's heuristic is that you need to get the "Very Disappointed" number to 40% of users to be at product-market fit.
A lot has been said that's critical about the customer development process. And I agree with some of it (it's moderately unhelpful on the Thiel path of 0 to 1 rather than 1 to n).
But Rahul's implementation is one of the best narratives about it that I've seen. A lot of Sunstone's seed stage startups use a similar process and we encourage any seed-stage company to consider using it. It makes progress measurable while you're still in prototype stage.
Measure what matters - this does.