Adding Commission costs on Quotes

On the Quote worksheet there is a box where you can enter a commission percentage to be added on to a quote. This however is not tied to a sales rep or any field in Customer Maintenance. We have certain accounts that we pay a rep a commission on and we want it to automatically add a percentage to a quote. I've tried using one of the Number fields on the Customer record but without heavy code writing I can't get the Quote worksheet to pull number in and drop it into the commission field. Are there any other fields already connected between the customer account that is pulled into a quote (Attributes?) that I could a rule statement of If this field is populated copy the information to the commisison field? (I know this is a very rough example)

Does anyone have this situation and how are you addressing it?