Roll Up a Roll Up Field

Our group uses 3 levels of decomposition, and we want to track completion at all levels.  For instance, the development team works off Stories (#3), which are decomposed from Epics (#2), which are generated by Ideas (#1).  I can track the percent completion of an Epic based on the status of downstream related Stories using a roll up field such that when a Story status changes to complete, then the Epic percent complete is increased.  However, I cannot further roll up the percent complete to the Idea level to see progress on the Idea.  I would like to be able to roll up the roll up field so that I can see percent complete at the Idea level for reporting to management as well as the customer.  The calculation would be slightly more complex, but still straightforward.  For instance, if there are 10 Epics, and 5 are 100% complete, 1 is 50% complete, and the other 4 are 0% complete, then the Idea percent complete should be 55%:

Idea percent complete = SUM((% complete of all Epics))/COUNT(Epics)
Example Idea percent complete = SUM(1+1+1+1+1+0.5+0+0+0+0)/10 = 55%