I Can't Model This Problem for a Dynamic Sheet

Hi all, I have found a flaw in a larger spreadsheet that I can't implement a solution for and is rendering the whole thing useless right now, hopefully someone can help.

I'm trying to allocate 'Customers' to Trucks after an optimisation model i have built runs.

Trucks can hold 100 units so I have built a compounded 'IF' formula that will allocate a Customer to Truck 1 if cumulative demand up to that Customer is <100, to Truck 2 if Cumulative Demand is 100< CD <200 etc..

My problem is that when a new truck is allocated I can't build into it a way to restart the count, so that as the Truck column has switched to 2 as the next customer's demand wont fit into Truck 1, a count starts from that customer until it's time to switch to Truck 3 etc.

Does anyone have any advice on what functions I would need to combine and could look up?

I've included an example of the data and existing formulas for this section of the sheet.

Thanks,
TKin