OK after reading more, I have begun playing with open jaw and stopover.

As a sample itinerary, I was able to price this open jaw at Saver 60k:

8/5/15 STL-JFK-MAD (6218 + 414)
8/19/15 CDG-DTW-STL (99 + either 1694 or 2623)


But when I tried to make MAD a "stopover" by adding a MAD-FRA flight on an intermediate date, the site always jumps the price up to 90k (essentially adding the 30k inter-Europe price). Which brings up this question....

How do you know, without calling an agent, what city is a valid "stopover" city, for another city?

Can you stopover in MAD then FRA?
CDG then FRA?
FRA then MAD?

I guess you can see what cities I am focusing on.....but how does one know what cities work as a stopover pair?