I agree with PJ and Bruce. Wait long enough and you can do about anything you want over the top of Future or another acrylic clear coat.
That said, what I ususally do is use Alclad clear. Not the water-based stuff, the real deal Alclad II Klear Kote. It comes in 4 levels of shine: Gloss, Light Sheen, Semi-Matte, Matte, and Flat.
My reasons I usually do this, despite agreeing with PJ and Bruce are a) I feel I don't have to worry if the Future (or some other underlying water-based clear) is fully cured, b) I feel the solvent-based clear is more durable (my opinion, I can't verify that), and most importantly c) the Alclad ALC-314 Flat is the flattest and prettiest flat finish I've seen.
Here is a link to the Alclad site. Caution, Alcad clears can be confusing. The stuff called "Klear Kote" is solvent-based, the "Aqua Gloss Clear" is water based and only available in gloss.
One more thing, if you are still hanging in there on this too-long reply; I pretty much ruined my second model back into the hobby years back with that foggy (Milky) effect you mention. I can't remember what product I sprayed, but I think it was a flat acrylic and most likely Vallejo Flat varnish. It was after that I bought the Alclad and I've been tickled pink with my flat finishes since.
Oh, and whatever you choose for your flat coat, remember, light, dry coats. Heavy wet coats can cause that "foggy" (miky) effect with Mattes/Flats.
Sorry, that got way to long.