Throw the Flat Base out... There's really no reason to even have it on hand. There's a number of clear flat, satin, and gloss overcoats out there in rattle-cans, in laquer, enamel, and acrylic... No mixing, no clean-up..
I use Testor's Glosscoat and Dullcoat along with clear Gloss, Satin, and Flat acrylics by Tree House Studios. Always test your clear overcoats on a paint hulk before you apply them to a model though..