I am making a simple inventory system which keeps track of units in and units out and then calculates the on hand quantity. So I have 2 fields: Units in and Units out. There are also 2 layouts, one for scanning in UPC's and another for manually entering part numbers. Everytime you scan a UPC, there is a new record. And when you enter a quantity manually the units in/out fields will get the quantity.
The calculation I am using for units in is this: