StillLearning has it right for PLANNING fuel for a flight, but that does not apply once you have taken off. Once airborn, actual fuel on board and what you think you can do with it is king.
A common question: Can one burn into the 45 min reserve fuel? If one thinks no, then why is it required to be there.
Answer: It is a planning tool that allows options for unplaned delays or deviations. Therefore, burn it if you come across any unPLANNED delay.
Again, there is a difference between planned and actual.
As I see it, there are (at least) three levels of ACTUAL fuel planning:
1- Legal - FAR's say one must cary enough fuel to make it to the destination (or alternate). Flame out on the runway or flame out at the gate is up to you.
Example: zero at destination (or alternate).
2- Personal - Just because something is legal doesn't make it safe. Just like those above, I personnally want enough fuel for at LEAST one go around (maybe more depending on the weather).
Example: 1000 lbs
3- Company - An airline or operator might defined the minimum fuel on board at the destination (or alternate if required) which will often be more fuel than the two options above. This increases the safety margin, lowers liability, but increases the possiblility of a devert if delayed. It is the business decision that they have made, so stick to it. For my airline it happens to be the reserve fuel number.
Example: 2000 lbs
I hope this helps. Please feel free to add your thoughts...