Populate a field only if there is data in another related table...?

I have a table that has an invoice date (tbl_qa).  I have a table that has an invoice date (tbl_invoice).  They are related through the ProjectID.  The tbl_invoice has a field called FinalPercent.  Both tables feed a seperate form (frm_qa and frm_invoice).

The user wants the qa to auto populate the invoice date ONLY if they put something in the FinalPercent field of the invoice table.

So if (of course they use forms) they put in 1/30/2013 in the invoice form and do NOT enter anything into the final percent field, nothing happens.  But if there is a date in the invoice field on the invoice form and then later on (or at that time, depends) put 75 in the final percent field, they want the invoice date on the qa table to populate with the same invoice date.

The qa form will always be open when the invoice form is open.

What is the best way to do this?

Thanks!