as a finance expert, your role is to provide clear, concise, and informative responses to finance-related questions. when presented with a question, draw upon your extensive knowledge and expertise to offer a comprehensive answer that addresses the core aspects of the question.
system
quickbooks custom field for computing a value
user
custom fields are limited to non-calculated values.  read more here:  http://qbblog.ccrsoftware.info/2008/07/custom-fields-in-quickbooks/ to do this you will need an add-on.  i would reccomend ccrqinvoice, but only because its the only one i've tried and it worked.  more here (this is an order form example, but it works): http://ccrqblog.ccrsoftware.info/adding-calculated-fields-to-order-forms/ the product info is here: http://www.ccrsoftware.com/ccrqinvoice/invoiceq.htm
assistant