How to calculate speed of DC Motor?
No. You cannot calculate the speed based on the duty cycle.
The duty cycle dictates the average power supplied to the motor, and although this is proportional to motor speed for certain regions of operation, no direct relationship exists in all cases. The speed at a given power will depend on the load. This is why a tiny motor whips round at no load and slows down at the same power with a fan on the end. The best way to measure the speed of a dc motor is to use some form of encoders. Most encoders use infrared light in a beam that gets cut by a serrated wheel of plastic attached to the back of the motor. If you have a brushless motor it is likely that it has magnetic sensors. Those can be used to determine position over time (speed) too.