Formula That Adds Count But Saves Count So Can Clear Field

I want is a field (e.g Large Parts Used) where I can enter in a number, then basically this number is subtracted from current stock field for Large Parts so I get an updated field of current stock on hand.

But what I want to do is once I've entered the number in the Large Parts used field, I can then clear that field but have the corresponding Current stock field to maintain what was last enetered.

E.g

Large Parts Current Stock = 50

(enter in) Large Parts Used = 2

Large Parts Current Stock = 48

(Clear field where 2 was entered into Large Parts used)
(Field still stays at Large Parts Current Stock = 48 although field where 2 was entered was cleared, so need it to save the information so can continually clear and re-enter amounts and have the stock continue to reduce)