Usually, 50/50 share is a bit offensive to artists, and for a good reason. Artists do a bit more than writers(sometimes a lot more, it depends on what kind of responsibilities each side takes upon themselves), so 60/40 share in favour of artist is considered something of an arbitrary "golden standard". Suggesting 50/50 split can sound like you're not an experienced collaborator, and thus not a good partner for an artist/writer collaboration.
Well, I'm mostly just telling you how your post MIGHT look based on the information that you've given. You'd also need to give way more information on your story, summary, what you're expecting of the artist and what you can and can't do as the webcomic writer. If you can't share these things with your future collaborator(or at least make it all VERY appealing to read), it's very hard for an artist to take a chance on you. Think of it as a show of respect - as long as you don't dutifully make a well-written pitch that would appeal to any artist, you have not learned to write well enough to work with an artist(generally speaking, as rare few people start early and fare pretty well). Collaboration is a game that is ALL ABOUT mutual respect, and at this moment you might need to offer your possible future collaborator more respect than what your initial message implies.