Velocity fluctuation
Velocity is a metric representing the amount of Product Backlog effort turned into a releasable Increment of product during an Iteration (i.e. Sprint) by an Agile Team Team (i.e. Scrum Team). Velocity average should tend to a constant number with the pass of the iterations but some deviations may be accepted. When establishing upper and lower limits you can detect when an Agile Team is “fluctuating” more than what it is desired.
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Hints!

Upper fluctuation limit
The maximum variance percentage accepted above the average velocity. 20% could be a reasonable number to start by. 
Lower fluctuation limit
The maximum variance percentage accepted below the average velocity. 10% could be a reasonable number to start by. 
Iteration's completed effort
Provide the effort completed in your previous iterations (i.e. Sprints) to calculate the average velocity. 
Now go and try it!