I found this on the net,

Exit efficiency = maximum possible difference in prices for this exit/profit potential.

for long trades ;
Exit efficiency = (ExitPrice - LowestPrice)/(HighestPrice - LowestPrice)

for short trades ;
Exit efficiency = (HighestPrice - ExitPrice)/(HighestPrice - LowestPrice)

A good number is 60% or more. With this calculation we can analyze whether our exit strategy need an improvement or already "just fine"