A $3$-phase, $415\; V$, $4$-pole, $50\:Hz$ induction motor draws $5$ times the rated current at rated voltage at starting. It is required to bring down the starting current from the supply to $2$ times of the rated current using a $3$-phase autotransformer. If the magnetizing impedance of the induction motor and no load current of the autotransformer is neglected, then the transformation ratio of the autotransformer is given by ________________. (round off to two decimal places).