You can check the fan speed versus voltage by testing them with a variable DC power supply. But I understand that some brushless motors may not work properly with PWM, so you would have to experiment with that.
If there is a problem, then you could run the PWM signal through a large inductor to smooth the PWM signal into DC. The higher the PWM frequency, the smaller the required inductor.
The duty-cycle of the PMW does determine the average output voltage. Just multiple the duty-cycle by the input voltage, as you did.