The way I, as well as most other professional songwriters, split songs with co-writers is not based on the number of words we divide to get at a percentage. The truth is, I've written songs with people who only added one line, or had the basic story which we wrote a song about, or had a partial title or first phrase of the song, or just helped with the bridge I was stuck on. I've also co-written when I myself only contributed one line!
Who's to say what makes a good song into a great song? Sometimes a very little part of the song is actually the most important part. So I just divide the song equally among co-writers.
However, this is just the writer portion we're talking about. Publishing CAN and DOES get split unequally, for lots of reasons. Sometimes the veteran writer will take the time and professional risk to write with a novice writer if the veteran gets the publishing, or a share of the co-writer's publishing. Sometimes it has to do with who is paying for the demo. Sometimes it has to do with who has a major publishing company and how much share they need to really pitch the song. Sometimes you share your publishing with an artist, label, producer, manager, etc for political reasons... to get the song cut by a major artist. There are many different acceptable business deals with publishing.
But writer's percentage is a sacred cow for professional writers. This percentage is paid by ASCAP, BMI or SESAC, according to each writer's affiliation, when the song gets released and played on public media.
If co-writing with someone leaves you feeling too unsatisfied as to each person's contribution, the answer is to go ahead and split the co-write equally, then decline to write with that person again.
But be careful... sometimes a person can be a worthy co-writer just by their presence in the room, if they tend to make you open up and create. This contribution is priceless.