Linking two spreadsheets for inventory

[Ralph D via OfficeKB.com]

I'm creating an inventory based master list for computer deployment, that I
will be linking cells that will be running a formula that will calculate
based on current product received and then the second part of it will link to
a spreadsheet on a shared drive(this spreadsheet is where we indicate what
type of computer desktop or latop and who it was deployed to) so we have a
running figure on how much should be on hand. I do know how to create a
formula that can link to the shared XLS file, but the cells in the shared
have X's to indicate which type of computer was deployed, each Row is going
to have at least on marked, what is the best way to write the formula to
indicate that if the cell is X'd then it subtracts one from the master
inventory list. Thanks

Ralph