Also, I figure that the Force vs Spread curve is not a straight line, as someone has already pointed out in this thread.
A graph showing a force vs spread curve would be a more useful measure of flex than a single figure.
Plotting force along the x-axis and spread along the y-axis, most flex nibs would probably start as a steep curve, levelling off to a horizontal line at the limit of their flex. The steepness of the gradient at a particular spread would be a measure of the flexiness at that spread; the point at which the gradient starts to flatten would indicate the limit of the flex, and would also be the point at which to stop the test, to avoid damage to the nib.
Looking at such a graph would instantly show you what range of spread the nib can achieve, and how flexy the nib is at any given point along that range.