I need to write a formula that uses the previous record value to perform a calculation and then has to reset based on a change in product number. Here is the data I have and need to work with:

ProductNumber
TargetDate (these are by month)
OnHandInventory
ExpectedReceipts
Demand

Each ProductNumber has 12 TargetDate's which I need to calculate the following for:

For the first date: ProjectedOnHandInventory = OnHandInventory + ExpectedReceipts - Demand
For the following eleven dates: ProjectedOnHandInventory = (the previous record's ProjectedOnHandInventory)+(the current record's OnHandInventory)-(the current record's Demand)

I then need for this to reset on change of ProductNumber, of which I have grouped on.

I appreciate any assistance I can get in getting this formula pulled together. It's the last thing I need to get this report off the ground. I really need this up and running ASAP.

TIA!

Kat