I haven't done this yet, however, from reading on here about using future, the norm seems to be to use windex, and make sure its the kind with amonia. You can thin it with that, and I have also been told, after you put the future/tamiya mix on, to spritz it with windex to help it loosen up and level out a little more. I think the mixing ratio is for how flat you want the future. Try this website, it will help you out more than I can, its worth saving to.
http://www.swannysmodels.com/TheCompleteFuture.html
Its a long read, but packed full of useful info! Good luck.