I use the flat for semi-glossing...
Trod on down to the local supermarket and buy a bottle of Future, then hop on over to your local hobby shop and buy a couple of bottles of Tamiya X-21 flat base.
Mix the two together, I generally start at a 4:1 ratio of Future to X-21.
Once you find that ratio that you're looking for, mix up a big batch (for consistency's sake), and spray away.
Gloss coat the model before decaling with regular Future.
This way, you're not applying dis-similar flat coats over each other, and you're less likely to get reactive results.
Jeff