50/50 on the composer's share would be fair, I'd say. Normally, in the traditional music biz, this part would be 100% due to the writer of the vocal line, but since the advent of electronic 'beats' this has changed, I guess. And rightly so. But the lyricist's share should remain with the lyricist, shouldn't it? You're not trying to cheat, are you? The musicians' shares again could be shared among the parties.
It's easy, but it's not THAT easy ;-)