to get an exact ratio yo will have to break out your math skills. you will need the compressed gasket thickness combustion chamber volumn and other variables. Your best bet to get an approx. ratio would be to call the piston manufacturer and the will sometimes have a CR for a given head volumn. But it will ultimatly depend on the amount of dome on the piston.
that is a good question my repair manual does not say. if you have a head off turn it upside down with a piece of plexiglass with a hole drilled in it placed over the chamber. Then get an eye dropper and measure how many cc's of water or oil or whatever liquid it taked to fill it up.