Thank you very much.
(How did you figure all of this out?)
EDIT: I hacked Profile.dat to get my Polar Star on Lvl.12, and this is what happened:
According to Noxid's formula, it should be 2*3 + (12-1) = 6+11 = 17.
17 is the id for a blade slash, but it shoots Lvl.1 Snake projectiles.
My guess is that the game converts any unrecognized weapon level into Projectile 1, i.e Snake Lvl.1
So what about Noxid's formula which seems to break down for me? Is it wrong or am I doing something wrong? (Look at my username)