If you wind 'out' (-) the rebound adjuster until it stops then 'in' (+) to the first audible 'click' - that is the starting point. Should be about 14 clicks from there to all the way 'in'.
The important part is; are the left and right forks set the same, as the rebound adjustment is set mechanically, by setting the rebound rod distance in relation to the preload adjuster, inside/underside the fork cap... That ensures the left and right forks adjust the rebound to the same 'feel'.