I literally just asked this question the other day, ha ha.
I didn't get a definitive answer as everyone's tuning board and preference is different I think. My best guess is just enough for the car to roll. I ended up going 1/4 of an inch, maybe a little more. Also a good idea to level it so the cars will roll straight. I did this by placing a marble at the top, watching it roll down, and shimming it up until it went in a semi straight line to the bottom.
I don't think that the degree of angel in the tuning board is extremely critical. Consistency in the roll is the key. Don't set it up so that the car is screaming down the board. The call it a slow roll for a reason. LOL.