If name = one in the list then x - y; Help me please!

Hi guys,

I'm doing this Process/Stock sheet and I'm trying to find a way to subtract what ever was used from stocks. For example: 10 boxes of type 'A' were used - stock is 20. 30 boxes of type 'B' were used - stock is 40. Sounds simple ,but the problem is there is so many types that I had to make a drop down list and from there I wanted the values to come up automatically, but I don't know how to get a field to recognize witch value has to be taken. Please Help Me!!

Thanks!