To measure what's working for B2B YC companies, especially over the course of two years period you need to answer the following questions:

    a) what are the growth rate when measuring customers which are *not* YC companies?
    b) what is the churn rate for that same group.

Measuring by Series A (Assuming investors are the compass not the users - aka the market), is completely anti-YC the way I perceive the YC philosophy from afar.