There are lots of interesting details in Lucinda Southern's piece on the Financial Times hitting 1m paying readers. This, in particular, is interesting:

A new metric, called Quality Reads, shows the percentage of page views where the reader has read at least half of the article, estimated by time on page, scroll depth, and what it knows about how subscribers interact with similar content. The news desks then get weekly, desk-by-desk analytics reports.

That's a smart iteration of the standard “time on page”-style metrics.

As always, if you want to make a paywall succeed, you need to pay deep attention to how your paying customer behave on site.