I have a 4’ tunning board ready to go. What height should the top be?

I have a 4’ tunning board ready to go. What height should the top be from the ground?
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 read once on here between 1-2 degrees. I shoot for 1.5 degrees. But please remember I am NOT a expert.