I agree it's hard to give a good answer to a general question, other than general answers. Like final reserve fuel, to fly at green dot speed in clean config for 30 min at 1500m AGL, or more must remain at landing, alternate or destination. Once you've used contingency and extra fuel, and all you have left is alternate and reserve, you should already have made sure that alternate fuel will be enough to land, whether you divert or not. If what is left won't be enough to land with final reserve at destination, you divert, before you start using alternate fuel. When you avoid mayday calls and minimum fuel alerts on ECAM you do it right. But that is rather obvious and maybe not the answer you are looking for. And it's the very minimum required.