Ratio....humm...I have never measured. Since I use acrylics, I make the wash fairly strong, wet the ground work with a spray bootle and drip the wash on with a plastic dropper. If the effect is too dark, I hit it a time or two with the spray bottle to thin it down and then thin the wash some more. I usually use a black-brown wash with just a hint of Prussian Blue, which keeps the effect from becoming too monotone and drab.