Airport upgrades - how many miles?

Good evening,
I've seen a few of you mention that you have occasionally been successful in upgrading at check in on the day of travel using miles or miles+money combination.
Are there any set amounts of miles for upgrades at check-in? I assume it would be more miles from a K to J rather than a W to J?
Do the standard rules of being in a specific fare class still apply?
It would be interesting to hear your experiences of where you have been successful or advised of an amount of miles which you deemed too high. I'd like to ensure an adequate 'bank' of miles just encase!
Thanks,
Luke
I've seen a few of you mention that you have occasionally been successful in upgrading at check in on the day of travel using miles or miles+money combination.
Are there any set amounts of miles for upgrades at check-in? I assume it would be more miles from a K to J rather than a W to J?
Do the standard rules of being in a specific fare class still apply?
It would be interesting to hear your experiences of where you have been successful or advised of an amount of miles which you deemed too high. I'd like to ensure an adequate 'bank' of miles just encase!
Thanks,
Luke