Process Cycle Time

I am a greenbelt with a project to improve a transactional process.  The teams goal is typical, reduce the amount of time it takes from process start to process finish.  I am having difficulty-calculating PCT as WIP/Exit Rate.  According to training guidelines Exit Rate is equal to the bottleneck or time trap operation, and work in process is considered all transactions within the process.   The problem is that both are variable rates in this process.  Should I use averages or perhaps concentrate exclusively on cycle time?