Common Sense
Good Question.
For me there is 1) What is legal ?
2) What is safe ?
There is common sense, experience,route experience and local knowledge.
3) One usable runway at destination. OR
Two or more seperate runways. If one is blocked it will no
4) How accurate are your flight plans ? Jeppensen ?
Is there a fudge factor with extra fuel built in or not /
Performance factor ?
Rome FCO I take traffic contingency fuel I may have to justify this by taking fuel to a more distant 2nd alternate.
Quite often when weather is good ( JAA) Final reserve + GA + Alternate.
I take 6-10 minutes extra holding fuel
Now if I have to hold traffic delays at destination ( less than 6 hours ) and the destination has two separate runways and VMC-good WX
If I have to, rather than going to Alternate. I look at No Alternate fuel
Approach can be made from MSA in VMC.
I say landing at destination is assured.
Dont like it but it is better than unnecessarily going to alternate.
If it were to come to expect on landing destination Final reserve + 400kg (A-320 ) I would declare a PAN PAN