5%=en-route continegency.
-weather avoidance requiring deviation from route
-less economical FL
-Higher forecast winds.
Min takeoff fuel at departure should have you arrive at destination able to make an approach, missed approach, diversion, and further approach.
If you land at you destination, and en-route, all went as planned, your arrival fuel should be-
Final reserve allowance
Alternate fuel
Contingency
If you arrive at your destination, and hold (because of an unexpected delay, for instance), but have an assurance that an approach and landing can be made, to not land with less than final reserve fuel, then the alternate fuel can be used to remain over the field/holding fix ready for an approach. An approach time would be good in this instance to calculate the fuel required in this case.
If you arrive at destination, and an approach cannot be made, then you should have fuel enough to go to your alternate. More is nice. But the diversion fuel will get you to diversion, and you should still have final reserve on landing
If at any time you are expecting that you will begin to use the final reserve fuel, then a PAN call is needed.
If you actually start using your Final reserve fuel, A mayday call is required.
Hopefully I got that right.