Spotlight on SQL Server Enterprise obtains the values from SQL Server using:
A packet is declared incorrect if at least one bit is erroneous.
See below Wikipedia link for more information.
Spotlight on SQL Server Enterprise calculates our rate as follows:
Spotlight on SQL Server Enterprise do 2 collections and take the difference between them (second collection - first collection) and divide by the time difference.
Here are a few links to information about bit and packet error rates, etc.