Fair enough - but if that usually works as a rule-of-thumb, it presumably is almost a 'co-incidence'. Start-up ('inrush') currents of large motors are typically at least 5 times the running current, sometimes 10 or more times the running current. Hence, if the cable is 'rated' (CCC-wise) for 'just' 1.5 times the running current, it presumably is almost a co-incidence' that that usually ensures a small enough VD when 5 to 10+times the running current is flowing.
Perhaps one of the greatest potential flaws with that rule-of-thumb is that it takes no account of the length of the cable, whereas the 'additional voltage drop' during start-up will be proportional to the length of the cable.
