I'd start with the rebound all the way fast and ride off a curb seated a few times. Gradually add clicks until the bike feels like its not pushing your backside off the seat, then speed the rebound back up a few clicks. This will give you a good starting point. It is generally better to run your rebound faster than slower. You can accomodate a faster rebound with your body, but if the shock is packing down over successive hits, then there is nothing you can do from the cockpit.
If you need to run your rebound almost all the way closed, then there is probably something wrong in the damper. Give the shock a full rebuild and start again.
Also, the way the rear suspension feels is very dependent on how you've got your fork set up. Too much rebound damping on the fork will require too much on the rear to keep things feeling balanced. Try to dial your fork rebound in with the same method, starting from full fast.
Note: make sure you run the fork and shock in the "open" modes to set the rebound properly (and set the sag% before you dial the rebound in - a different air pressure will require a different rebound setting).
