This is an age-old question....

- what is the definition of "Productive Work Time"?
Typically, you would want to know how long the Agent had been logged in - and how much time they were available to take calls - and how much time they spent taking calls - and how much time they spent "not ready" to take calls...
So, use the following as a guide...
TotalLoginTime = amount of time Agent was "on shift" (T%)
TotalTalkTime = amount of time Agent was "working" (x%)
TotalTimeNotReady = amount of time Agent was "not working" (y%)
It should follow that x% + y% = T% - however there will be a "gap", which (depending on how your Agents operate) may include some of the following;
TotalTimeACW
TotalTimeRinging
TotalTimeDialling
Hope this helps?
TT