<...> I can say that it is less than 910 and more than 755, but what exactly is the number?
You can’t really tell which is your exact activity magic number, and even less if you reached your Activity requirement before Merit requirement. Nevertheless, there are two infallible methods:
One is to find out what the forum’s secretSeed is (used to derive all personal Activity magic numbers):
<…>
update smf_members set ID_POST_GROUP=21 where ID_POST_GROUP=8 and
activity>=775 and activity>=775+conv(substr(sha1(concatenate(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.
note: In order to see the code, I had to change 'concat' for 'concatenate'. Follow the link to @theymos' post to see original code (Cloudflare is blocking my post if I use 'concat').
The second is to ask @theymos to reveal your Activity magic number (I’ve seen some posts here where he has done it).
The code in @theymos' post basically adds your userId to the forum’s secretSeed, performs a Sha1 over them, and then converts the first two characters returned from hex to decimal, adding the result to 775 (thus the 775-1030 range).