Ask HN: What milestones do you use for vesting equity of minority co-founder?

I'm looking to bring a front-end developer on-board as a co-founder with equity compensation (5-10% total equity). I want the equity to vest 3-4 times based on achieving 3-4 milestones (i.e. 1% for achieving M1, 1% for M2, 1% for M3 and 2% for M4).

What milestones have you successfully used to tie the vesting to?

How have you structured the vesting for part-time participate (that will probably grow to full-time as the business can support it)?