Actually the correct cable length at the small chain ring (FD) and cog (RD) positions is the length that gives a taut cable with no slack but with exactly zero tension, the length that if it were just one milli-smidgen shorter would cause tension. The idea is to have the derailleurs in perfect position for the smallest cog (RD) and smallest chain ring (FD) due to the high and low limit screws, respectively, not due to being pulled on by the cable. Then when one unit of shifting is applied to the shift lever, the cable instantly pulls on the derailleur. That is when the tension in the cable is supposed to start. It is an ideal that is hard to accomplish, but that is the goal.