Button Formulas Help

I am creating a stock record database for all the goods stored in our warehouse. I have two fields that I am having difficulty with. I have the first field "Stock (In Single Items)" as that is how our system in work counts the stock on the shop floor. The second field is "Case Quantity". This is in a plastic wrap / box sent in by the manufacturer, how many products are inside each box / wrap.

I have created two buttons "Add Case" and "Subtract Case". This will add the number from the "Case Quantity" field to the "Stock (In Single Items) to make it easier and quicker to input the figure to save having to delete and add the new figure.

Was also wondering if instead of a number field for the Single Stock field, if I could make it a formula field and still have the button principle as mentioned above plus with the addition of another two buttons saying Add Stock and Remove Stock which would prompt up an input window asking how much additonal stock to either add or subract without having to maually delete the figure in the Stock Single Items field?

Many thanks Peter