Stock monitering

[Eye Specialist]

I wonder if someone could suggest a simple and good way of monitering stocks in my business?

I have one file that has a table of invoices with the amount of items going out. The items I purchase is on another file. How can I get the amount in stock to be automatically calculated each time I issue an invoice of the outgoing item? I assume I should use the calculation field or do I need to write a script for that? I'm quite in the dark about this. Help is appreciated. Thanks.