Assigning percentages in a band situation is the worst kind of politics ever. Having been in a couple, and observed how other bands so it, here's my take:
If you bring a (nearly) finished song to a band, by all means insist on keeping the writing credits and royalties. But if your song emerges from everyone playing together in the room, even if you create the melody and the words, an even split among everyone is the way to go, especially if you get a sniff of success, and want the band to last more than a couple of years.
Music is (generally) better when its collaborative, and I think distribution of credit should reflect that. Sometimes, the way a song is played is as important as the song itself.