Why not just use the standard four year vesting with a one year "cliff" for the full 5-10%? If your co-founder isn't a good fit or isn't hitting milestones you can part ways early and retain all or most of the stock.
More on founder vesting here: http://cdixon.org/2009/04/21/founder-vesting/ http://venturehacks.com/articles/get-vested-for-time-served http://venturebeat.com/2010/01/04/ask-the-attorney-founder-v...