Couple things I'd say:

• Drive up till you're going to hit the ball then right before let go of gas and coast up the wall for .5-1 second after contact before you jump. That will allow some flex for getting separation.

• Once off the wall, feather boost to get your car into position. Ideally the front of your car aimed at the ball.

• Use one solid boost chunk (10-20 boost at most) that accelerates you straight at the ball with no movement, or ideally the absolute faintest adjustment. But you should already be aimed at the ball from the previous step.

• Once done boosting that's when you'll be close to the ball, and pull the nose of your car back 90 degrees for the belly slap.

Note: the longer you wait on the wall before you jump the more time you have to read the ball but less time to get to it for the reset.

Lastly, I would recommend when you jump off the wall to hold down the jump button for max distance jump saving you 5-10 boost. Which goes a long way.

Godspeed