Ability to Bill Partial Amounts on Groups by % (not fixed amounts)

I know there is a fixed amount option, but my accounting department bills partial amounts for groups. For example for concrete work, currently all material and work grouped under 1 but have pour concrete first (Which we bill for) and the rest of the work done several days later. Our accounting department would have a hard time to keep track of what partial amounts have been billed for a separate group.