It also depends on what look you are hoping for. The most controllable way to paint (IMO) is to over reduce, and build tones and hue by layers, and minimising overspray. But there are times when you may want more immediate vibrant colour and you need to reduce less and increase pressure to suit - remembering to account for any overspray.
There are too many variables for a universal reduction recipe, and as mentioned weather also plays a part. Different colours too, more heavily pigmented colours may require more reduction than others.
Start with a drop of paint, and add a drop of reducer - spray, and add another drop of reducer etc, until you can spray a clean crisp unbroken line, and a satin smooth filled area. More reduction = less pressure. Spidering = less pressure or less reduction depending on desired colour density, grainyness = more reduction or more pressure depending on desired colour density.
This then becomes your base mixture (which will probably be refined with experience). Then you can adjust if needed depending on colour or conditions. Hope that helps.