I don't pretend to be an expert, but I think it 50:50 with water, with acceptable results. It's just the way I was taught to do it, and I suppose I am subconsciously concerned with the notion of unthinned Future gumming up my airbrush. I have heard some folks actually thin it 50:25:25 with water and rubbing alcohol, which supposedly gives a smoother finish, but I've never tried it.
Liam