How to calculate and measure motor constant for stepper motor?
Almost certainly you have misunderstood your k_e measurement. Your shaft speed is almost certainly in rpm rather than Hz, so your values need to be multiplied by 60 to be accurate, and 60 is your missing factor.

The reason I say "almost certainly" is that, for instance, your 11.1 V/420 Hz is equivalent to 25,000 rpm, and that's pretty impressive for a home setup. Not to mention unrealistic.