TomN
you are a wise man. A certain viscosity feel, rather than a standard ratio is the way to prepare your paints for spraying.
The rule of thumb for preparing paints (or other materials) for airbrushing is to reduce them to the approximate visual viscosity of 2% milk. As starting paint viscosities often vary from color to color, even within a specific paint brand, it is best to avoid fixed thinning ratios. It is also best to vigorously mix/stir paint rather than shaking it before use because mixing/stirring paint better blends pigment and base creating a more consistent paint from the top to the bottom of the bottle, and causes pigment to re-settle slower. Various paint types and materials, including acrylics, lacquers, enamels, urethanes, inks, can be applied with an airbrush if properly reduced for spraying.