Founder's contract for a nights-and-weekends effort?

Founder's contract for a a nights-and-weekends effort? My friend and I want to bootstrap a startup as a nights-and-weekends effort. The first thing we want to do is to agree on a contract that will keep things fair if one of us doesn't contribute as much as the other, or bails out completely. After reading HN threads on founder disputes, we have so far come up with the following. Please critique, who knows what trouble you might save us down the road with your suggestions.

Our agreement so far:

1. 50/50 ownership (subject to vesting)

2. Four year vesting schedule (25% at 12 months, 50% at 24 months, so on...)

3. A commitment to work x hours per week.

4. Some rule where a founder who fails to meet that commitment for a prolonged period can be fired.

5. Eventual compensation, at some percentage of market consulting rate ($50/hr? $100/hr?) from revenue for the founder who worked more, if the other founder fell short of the commitment.

This may seem pretty complicated, but our goal is to set something up that will ease tension rather than create it. Any comments/ideas?