MTTF

The simplest way is usually the best and what you describe is that. Of course failures are not always down to a simple break, but it's a start and the flag tag can always be refined.​

I would agree with (Missing Content)​.

In your question you ask "easiest, fastest and most reliable way?"

I would say yes to Easiest and Fastest, but I would say No to most reliable. As you don't get any detail into the type or reason of the failure. In fact you can only say that it is not running, you cannot determine if it was a planned stoppage or a breakdown.

For a more reliable solution to calculate KPIs such as MTBF, MTTR, Availability and Reliability, there are more involved solutions. One that I have experience with is RtDuet, a downtime tracking software that is built on the PI system. RtDuet from RtTech Software

Does that give you essentially a manual interface to support operators entering in outage information and such? In general its the only way to capture it, you cant rely on info from the maintenance system much either unless things are really rigourous

use PI Event Frame. this is buit in feature.