The inverse focal length rule

I concluded that the rule of the inverse of the focal lenght has been adopted rather empirically, so there is nothing to hold on to this principle. Besides, it is not going to be linear, so it is not worth it to keep rigidly to this principle. Time to rev gave in the case of the calculator is 300 x 1.8 1 / 480 which means 1 / 500 exposure time.

But I suppose as a rule of thumb it gives you good idea on when to shoot from hand and when to use a tripod.