It has been stated in several threads already, at the moment you register an account on this forum, the activity number in which you become legendary member at, is already set. Nothing can and will change that.
It simply can't be so in respect to Hero members that are about to become Legendary (i.e. higher activity Hero members, say, above 900) for very simple reason - when their accounts were registered there hadn't yet been a Legendary rank on the forum. It appeared only in late 2014 - early 2015, if I'm not mistaken, while the total majority of such members (including me) registered in the fall of 2013. It could be that the date was set when this rank had been added, though, but I'm still not sure...
Could anyone clarify this matter?
Theymos has detailed how this is actually done. It depends on your id.
And of course, a secret seed
...
Currently this randomness is done with a single SQL query, which is very convenient.
update smf_members set ID_POST_GROUP=21 where ID_POST_GROUP=8 and
activity>=775 and activity>=775+conv(substr(sha1(concat(ID_MEMBER,
secretSeed)), 1, 2), 16, 10);
The required activity level per user is suitably random for betting, but anyone who can read my code (there are a few such people) will be able to exactly predict when someone will become Legendary, so I don't really recommend it.