That can also be quite easy calculated mathematically: the ball is 1 unit in diameter, so 0.5 in Radius. After some easy calculations: for a full rotation (2 pi or 360 Degrees) it should travel in X direction exactly 3.1515(pi) Blender units. After some more also easy calculations: with this ball size you have to multiply the travelled distance (in my case 5.93 Unit's) by 2 and you have a rotation in Radians.
For other sizes: rot = x/r,
x - travelled distance in X
r - ball radius.
