Formula Field on Sales Order Product Detail

Formula Field on Sales Order Product Detail

We have a need to add a calculated field at the product line item detail on the sales order. This is an internal field that doesn't need to be exposed to the customer. 

We have a number field on the each product called "factor".  When it is added to a sales order we simply need a field that takes the price / factor to calculate. 

Seems like it should be straight forward but can't seem to see where I can add a formula field at this level.