calculate avg value over time - alert if 200% increase

Hi,

I am trying to track a value on a backend server if a certain operation spikes to greater then 200% of the average value per 5 minutes, not sure how to do the alert part unless i enter a static value like this, and alert on the eval "high" value.

index="vip" host=ship*be* OR host=van*be* OPERATION="Validate" source="/app/logs/vipservices/vipservices.log" earliest=-5m | timechart span=5m count by host | eval BE_spike = if( count > 2000, "high", "normal")

what's the best way to schedule an alert if the OPERATION=Validate avg spikes higher then 200% of the previous values over time?