Percentages for songwriter and artist

by Tom
(New York)

Hello,

I have a song that I wrote and also recorded all the music and vocals on this song. I now have a singer that feels he could do something with the song. Leaving the song and arrangement as I recorded it,he would like to replace my lead vocals on the previously recorded song with his.

What percentage or agreement would be appropriate to give the artist if something were to happen with my song?

Thanks
Tom