737incog'.
There are the legal boundaries, which may allow you to takoeoff and 'give it a go'; and then there are the airmanship criteria, for which I think the pax pay us to uphold.
In the first instance it may be that destination is a long way off; indeed it might be home base to which you are trying to return. Not helpful to leave a/c & pax grounded far away with little help at hand for long delays. As said, in Europe there should be somewhere as a bolt hole; except remembering one dark night moons ago when everywhere, including the close continent, was out in FZFG. exept PIK & BUB. Since these are in opposite directions it was not helpful.
In the second instance, when ops want you to dispatch to an airfield XYZ which is not your destination ABC, this is what I do. Consider XYZ as planned destination, making sure you have at least the trip fuel for ABC. (It might be that XYZ is shorter nm than ABC.) Then take fuel for an ALTN for XYZ plus all the other bits. Thus you now have a safe fuel plan when you depart. Should ABC become suitable nearing XYZ then you can make an approach at ABC and you will have fuel to divert to XYZ should it fail. There is no complicated 'how much extra shall I take' etc. It is a simple straight forward fuel plan as any other flight, but gives you all the flexibility you need. Of course, you might want 2 shots at ABC, so enough for the second approach might be all the extra you need.
Before JAR, in the good old days, the fuel so required was enough to make an approach at original destination, have 2 ALTN's, G/A, divert to further ALTN, given that both ALTN's were in the same direction; hence my quip about PIK & BUB. Of course you would land at the nearer ALTN. That might have been company rules rather than CAA, but it made common sense. I do not think JAR has banned common sense; hopefully.