I like your second idea. This will vary so drastically from app to app. For some apps you might need the designer to make a logo and a few images (assuming you use out of the box icons). That could be a 95/5 split on effort. A game or super visual app COULD go 50/50 on time expenditure but that's probably rare. Also, depends on the skill levels. Should you assume that value per hour are equal?
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.