For a data cable, the calculation doesn't mean anything. First point is, you have no 10 MHz sine wave with ethernet signaling but a complex pulse pattern. Second point is, you have termination at both ends and (hopefully) not standing waves. Thus signal delay is the most interesting cable property, furthermore frequency dependant attenuation, appearing as pulse distortion here.
As mentioned above, this number means nothing usable in ethernet cable. In some point of view, it can be interpreted as an electric length of this cable, which is in this case 1,5*360 deg = 540 degrees or 3 pi.