Assuming 1 unit of speed = 1 units of diameter then I would have thought that the ball would need to roll about 79.62% of the way round, which would be about 286.632 degrees.
So I guess it would be either:
286.632 * delta
or speed*57.3264*delta
So I guess it would be either:
286.632 * delta
or speed*57.3264*delta