How much to thin Italeri Flat clear and Tamiya for airbrushing.

I tried airbrushing Italeri clear acrylic paint a couple of weeks ago and when it dried white circles formed across all flat surfaces. It also brought out a lot of remaining pigments on the surfaces. I brushed the excess off and then I airbrushed some Future over it again. I know I thinned the flat clear too much. Probably 4:1 water to paint ratio. So what's the better ratio to thin Italeri Flat clear paint before I mess it up again?

Also, on the topic of thinning paints and the like, can I thin or at least make Tamiya paint last longer for airbrushing when mixed with a bit of Future? And is it "proper" to thin Tamiya acrylics with water or not? I've done that with my sherman and the paint came out well.