> But all depend of the "measure" of the distance of your Control points "above"
> the curve: are they proportional of something or are they pure random?

That distance is not random, but it is not really a fixed distance for any single point, because the shape of the curve depends on more than just that one control point, it depends on the surrounding control points as well.

For example, if you leave that one control point in place, but move the one next to it 100 units away, the original control point will now be a further distance away from the curve. The curve at any one spot is affected by several nearby control points.

But the more control points you use, the smaller area of curve is affected by each point. So if you use a fairly large number of points, the accuracy will keep on improving.

- Michael