⭐ Merited by PowerGlove (1)
Can you make getting legendary provability fair? So we know if you don't like us that we get it anyway 
Nah, that'd significantly complicate things. Currently this randomness is done with a single SQL query, which is very convenient.
Code:
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);
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.