Calculations in MS Access 2000

I’m designing a database in MS Access 2000 to keep track of inventory and I can’t get a calculation to to give me a correct answer. The calculation (On paper) is as follows.
In_Stock=Current
Current=In_Stock-Adjustment.
Any ideas on how is the best way to implement this? Every way I could think of always gives me the wrong answer(ie things like 20-5=10). Thanks. JJC