i think you're supposed to have less than .5 degrees in the front
but in the back i think you can have up to .8 or 1 degree

as far as scraping, i can help you with that cause i'm slammed.
when you'er going over a speedbump or a ramp or something, always take it diagnaly, approaching the obstacle from the left and going over it in a diagnal/right angle

say that you're heading north and you come to a speedbump or a curb cut. you need to travel northeast over the obstacle.
that should clear it up.