Define a scaling threshold for Azure Web roles

Azure covers the concept of resilient scaling, and I was able to achieve this with work roles . However, when it comes to my web roles (e.g. MVC applications), I'm not sure what to control (or how) to determine when its good time to increase (or decrease) the number of running instances, I assume I need to control one or several performance counters, but I don’t know where to start.

Can anyone recommend best practice for evaluating the load of MVC Web Role instances regarding scaling decisions?