consider basing the combo rowsource on a query that references the main form control as a parameter and eliminates the values you don't want based on the main form data. You would have to deal with the potential that the main form control doesn't contain any data. Setting the values on form load won't help if you're navigating between records. Those notions make me think the combo and main form control are bound, which doesn't seem right. If it's possible to change the main form control data, the combo value can be out of sync. The idea of having to rebuild the combo rowsource every time the form navigates to a new record doesn't quite make sense to me either.
Another method is to use a value list, and rebuild the list in code based on the main form control, but I wonder if you shouldn't explain more about what's going on.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.