If you think Goku's regular full power against Raditz is 416, then it is closer to 2.25x. Raditz measured Goku's Kamehameha at 924 which is approximately 2.22x 426. But Raditz said the power was still increasing, so a 2.25x increase to place the super kamehameha at 936 is fitting.
The 1.5x suggested by others comes from a complicated scaling argument.