Pinhead
As you rightly say, discretion can (perhaps MAY to be more accurate) be used to extend a level variation FDP. However the discretion that MAY be exercised is itself modified by the variation permit.
I believe most Captains would be more than helpful in providing you with details. I frequently find I am answering questions on this and other FDP related questions with the Cabin crew. Having said that I am often amazed that they are so unwilling to read that portion of their ops' manual that deals with this subject. I should also point out that given the fact that Cabin crew can usually work longer hours than Flight deck crew, I fail to see how "..the drivers always like to put one on you." Remember that the ops manual has been approved by the CAA and you might save yourself (and the CAA) considerable time by making an attempt to understand it.