I was trying to repeat this exercise with a slower starting speed on the first slope, but I find it very hard to make the ball accelerate without it going above or below the platform it's rolling on. Is there a general principle to follow here, or is it just approximation in the graph editor?
Practice my friend ;)
That's what all these exercises are designed to do. Teach you to control every situation that you will encounter in the real world of animation, all while training your eye to see better.
Keep at it, you will discover your own little tricks and have some ah-ha moments.