I find www.itn.net to be the most valuable tool when planning mileage runs. To qualify for any special fare, the right kind of seats have to be available - whether it be K, V, Q, B, etc. At the itn.net website (if you have your profile indicating you are an "expert") you can see the various buckets. Just because the fare is published, i.e. SDF-SEA $128.00 round-trip, it means nothing if you can't get a "K" seat to qualify.
I slaved away on a simple STL-DSM ticket and the return (on Sunday) was "almost" impossible because I needed a couple precious "K" seats. Thanks itn.net for the help.