1) Do a calc of hours worked by everyone in the past

2) Split proportionately amongst those hours

3) Apply a wage rate to those hours depending on the job involved

4) Adjust the wage rate so earlier is higher (more risk), later is lower (as the business started working)

Use that as the equity split going forward that you have to still work to vest.

Using a mechanical way to attribute effort everyone can acknowledge is extremely important. The area you'll really have disagreement on is the wage rate to apply & how much premium to apply for an earlier vs late joiner. But those are easier conversations.