I think we are into thread drift again!
Fuel planning is just about the most in-exact science known to man because none of us actually knows what EXACTLY is going to happen.
For example, when I flew DC-10s for Laker many years ago, we used to carry an extra 1 hour holding fuel for JFK and ORD. Guess what happened; we would get stuffed into the Micke holding pattern for an hour when the weather was crap and still end up diverting.
I then flew the same aircraft for a Part 121 carrier and they had a "Plog Fuel" policy unless there were extenuating circumstances. I was very unhappy with this to start with but I got to love it in the end.
If I bowled up to Long Island and was told to take up the hold at Micke intersection for "at least 40 minutes", I didn't have to make a decision. I just asked for immediate vectors to BDL and landed.
This only happened on a very few occasions. The diversion caused the company an expense of a few thousand dollars but there were several plus points:
1. The punters cleared customs, immigration etc at BDL, and so, when we finally got to JFK we were treated as a domestic arrival so they could pick up their bags and go home.
2. We (the crew) were on the ground (which is a very good place to be when the weather at JFK is crap). We could now pick up lots of very nice fuel for the onward journey.
3. All the grey areas about how long to hang on in the hold before diverting have been removed.
4. The overall saving from the annual fuel bill makes it more likely that we are going to get paid next month.
On balance, I like the Plog fuel policy unless there are some other really serious considerations.