Ironically, the first clear flat that I ever tried was Floquil, and my experience mirrored your own; it wasn't truly flat. I then went to Dull Coat, but got irritated with inconsistent results. SOmetimes it was flat other times not, so I too decided to go the acrylic route. I first tried with MM Acryl Clear Flat and was not impressed. It seemed to dry somehwhere from satin to semi-gloss, certainly not dead flat.
Since the highy recommended Polyscale was not available locally, I tried Future flattened with Tamiya's X-21 Flat Base at about a 4:1 ratio of Future to Flat Base and am pleased with it.
1) It does dry dead flat, and you can adjust the flatness by varying the ratio of flat base to Future.
2) I've not had any issues with applying over washes.
3) As with normal Future, you can thin or not as you see fit. I thin mine a bit with Tamiya's acrylic thinner