I am speaking purely on opinion and supposition, so take this with a grain- no a sea of salt.
How much revenue share you should be looking at depends upon a few things:
- How much work you do compared to others (how important your work is to the project) and the quality of your work
- How much others are getting
- Costs involved; legal and development costs - if the designer needs to do PR, pay for legal, pay for distribution and everything else, they're going to need a lot more revenue share for that (or whatever other system you choose, like everyone pays equally and so on)
- How many people there are in the team
If the game concept is someone's IP, it is expected that they would receive higher revenue share.
If I were setting up a team, the two main components - coder and artist - would get the highest share. In a two-person team, I would expect 30-30 for share, 20% for costs and PR, 20% for future development (after costs). If you're the main or only artist, you should be looking at at least 20% from profits. Again, only my opinion, but you have ownership of your art, and if the lead artist leaves with all their work, development is screwed, at least until they find a new artist and completely reinvent the game. Less so if the designer can also create art, but still something they should consider.
If they are talking about revenue, it really depends on how much of that is going to be costs.
If you have any doubts, or even if you don't, I would suggest you see a solicitor.
I would ask for a full list of where each percent of revenue share goes, and then see a solicitor.
Edit: Reading over your post again, I think you should find out a date for when it will ship, and how high that percentage will go. Also, why would they be offering revenue share which increases when the game isn't even shipped? Is it early access/beta?