Agree with Tobias that you just have to ride around on different settings.
A good starting point would be 15 all around and keep it there for a few days of driving. From there you will can decide to go up or down on the stiffness, changing it a click or two at a time. Also at this point in time I would just keep the adjustments same front and back just to keep things simple.
I have 27/27 on my set up. It used to be 25/25 for a few months. Of course my taste on oscillation greatly differs
Another thing to consider is that it fully depends on your spring rates. If you build the coilovers with stiffer springs (higher rates) you'll want higher/more dampening. And for softer springs (lower rates), you'll want less dampening. It's all about balancing the oscillation rhythm of the movement.