Daniel:
We still allocate time internally based on percentages for each phase, but our internal historical data shows that we need to be very careful to adjust those percentages for each project. We rarely use a set percentage for billing because we've discovered that projects can be highly variable, depending on scope, unforseen conditions, and the client. For example, I recently completed a very large home remodeling and addition that took two years (!) of client handholding just to get through the design process. Can't put a fixed percentage on that!
Tom:
I'm not sure that I completely understand what your arrangement is to be, but generally I'd say, "Keep it Simple." You're worth what you're worth, and unless you're in a financial crunch and need immediate income, I'd just set a price you think is commensurate with what you're bringing to the table and with the effort you'll think it'll take, then sit back and let him react to it. It's a lot easier to negotiate downward than it is upward.
Don't be shy about asking for what you're worth. I once did some consulting for which I almost asked for $4000 a week plus expenses, but at the very last minute, what came out of my mouth was $3000. The client agreed so readily that I knew I'd left money on the table.
Modelhead:
Don't know if we'd be interested or not until we saw more details. There are just so many variables...