Hi,
I just thought I would write to add my support to Bealzebub. He is absolutely correct in what he writes. When I explain this subject to newcomers, I try to put things another way. That is you must first appreciate that there is in fact two sets of rules here. The first set covers 'Flight Planning' and the second set covers 'Reality'. Fantasy Vs. Reality if you like. While it may appear that your schedule is leaning more towards Fantasy than others, you will find that the Authorities will become involved if the discretion reports become too prolific on a certain route.
Once you have departed, the rules of reality superceed those of the fantancy. It's real time now!
Further more, there is absolutely no requirement that once a problem ensues that the whole flight has to be replanned from the beginning. For Example: I had to divert into Vienna due to a bomb threat. After the Austrians were satisfied and we were happy, we continued the duty even though it meant we would do three sectors. They tried to tell me I had done 3 1/4 hours into discretion because they tried to apply the FDP for three sectors. They were wrong! The planned duty was for two sectors and since no-one could plan what happened in reality, the duty time allowed, was for two sectors! However, the Captain has to make a decision that requires him to take into account all factors, this of course would include the effects on the crew doing a third sector. Had it not been for a very experience F/O being with me at the time, I might have made a different decision. Most importantly, the Captain makes the decision, it is his discretion no-one elses and especially not those back at base no matter what their position. If the company can't handle that then they are in the wrong business.
I hope this helps!