record calc that depends on summary

[Arthur]

Say I have one table with four fields as follows:

StoreID SignType Units ProductID
12 AA 5 168
12 AA 3 168
12 AA 8
12 D 2 105
12 D 1 106

Notice that there is no product for the third record.
I need to calculate a new field AdjUnits so that within
a grouping of StoreID/SignType, any units with no
ProductID will be added proportionately to the other
records:

StoreID SignType Units ProductID AdjUnits
12 AA 5 168 10
12 AA 3 168 6
12 AA 8 0
12 D 2 105 2
12 D 1 106 1

This seems pretty straightforward, but I'm new
to Access. Can someone help please?

Art