I am very similar to Doug E Style with how I choose my fuel. It’s a really good question that the OP has asked. One thing I have noticed is that the flight plan is normally within 20 mins or so of what I think is sensible but once every few months it’s either a lot lighter or a lot heavier than what I think is sensible. Sometimes more than an hour difference on a 3 hr flight. It is worth building a strong and repeatable method ( the same every time) of independently calculating what you think is a sensible fuel figure for those days when the computers don’t quite get it right.
For the OP: I would suggest you choose a method from this thread that you like and try it out for a month, jot down your independently calculated number on a piece of paper before the Captain has discussed their thoughts and then when you and the Captain are different by a significant amount, respectfully tell them in the cruise that you’re trying to develop a good technique and ask if they could explain their thought process. Most Captains would be more than happy to explain their view , be impressed that you’re trying to better your skills, and if they aren’t then you’re probably not missing much! Also keep in mind that everyone has different risk tolerance and that’s ok.
What aircraft type do you fly?