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You would have to take a programmatic approach to this: AFAnalysisService.QueueCalculation Method
This is a new method introduced in AF SDK 2.8.5.X that allows you to do recalculation from an external program.. For those who know how to program, it is relatively straight-forward to setup up a script/service/program to monitor the number of events, and force a recalculation based on a condition.
Dan's answer above is correct. There is no native way to do this in the AF Client (PI System Explorer) and you will need to code this using the AF SDK 2.8.5.X as Dan mentioned.