Then bill your time back against the revenue at realistic hourly rates for the qualifications and experience. Finally split the profit as decided above.
Why not simply count hours on both sides until 1.0 ships and then split by the ratio of used hours?
For instance, if I were to bring a pro designer onto a project that he was only doing %25 of the time on, I'd probably give him 50% cut (because his time is worth double what mine is). Ultimately, kind of a complicated question.