Thank you Khalid.  I've also just found the supporting documentation for the steps you mentioned.  Their walk through covers the creation the accounts (such as the "default income account") that are needed to make the invoice by percentage work. 

https://doc.odoo.com/7.0/book/1/1_3_Real_Case/1_3_Real_Case_db_setup/#configuring-accounts

Hello,

In order to create a global income account and use percentage invoicing, you have to go to Settings -> Technical -> Parameters -> Configuration Parameters, then create a new field with parameters as follow:

• Name: property_account_income_categ
• Field: Income Account. Be careful! There are 2 fields with the same label "Income Account"! you have to choose the one corresponding to the field property_account_income_categ. Use the Search More functionality in the dropdown.
• Type: Many2One
• Ressource: (empty)
• Reference (first unlabeled dropdown): account.account
• Reference value (second unlabeled dropdown): your default income account, for instance X2001 Product Sales

However, in the generated invoice you will have only one line corresponding to the advance percentage. Be sure to also print the original document (SO) for your client in ordrer to have the details of the invoiced products.