I know the formula of duty cycle is

D=Vout/Vin

D=duty cycle

Vin=15V

Vout=5V

therefore the duty cycle is =5/15

i.e 1/3

therefore duty cycle in percentage is 33.33%

is my calculation right or whether I have to consider the speed of motor at 15V(2000 rad/sec) and at 5V(400 rad/sec)? If yes then how?