Based on your financial expertise, provide your response or viewpoint on the given financial question or topic. The response format is open.

Quickbooks custom field for computing a value
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