I use that combination all the time and haven't had any white blotches. I never thin Future at all. I spray it straight from the bottle, even with XF-21 mixed with it. Mix the flat base very well in its bottle, add it to the Future, mix that combination very well, and spray it.
Do you have a moisture trap on your airbrush setup? Could be you blew a little water out with it. If so it will probably eventually dry OK.
What kind of paint is underneath the Future? If it's enamel you can remove the Future with Windex (spray some on a cloth and wipe the bad spots off the model). If it's acrylic and is thoroughly cured you can probably do the same thing, but be very careful since Windex can remove some kinds of acrylic paint.
Another light coat of Future might clear up the white spots to.
I usually use about 3 or 4 to one for a flat finish. For semi-gloss, maybe 5 or 6 to 1. A lot of it depends on what the surface underneath is like to. With a flat paint finish it will probably take a higher ratio of Future to get a semi-gloss finish. Spray some on a piece of scrap and play with the ratio until you find one that you like.