Declining motor torque with lower voltage.

I found this statment googling around today.

"It is common motor knowledge that motor torque declines by the
square of the change in supply voltage."

First...Is this true.

And second if true would this be the proper way to appy this.

1) You have a treadmill motor rated 130VDC, 2.5HP 6700RPM, and
373 oz-in continuous.

2) The motor is run from a 80VDC power supply.

Well, the change in voltage is 130VDC - 80VDC = 50VDC.
The square root of 50VDC is aprox 7.
So the motor torque declines by 373 0z-in / 7 = 53 oz-in

???? Is this right ????

Wally