SetDCPWM1 help

i have to design a robot using C18 C compiler library code to move forward
In the code, i dunno wt number should i use in DCPWM1(duty cycle)
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For opentimer2 : T2_PS_1_16 & T2_POST_1_8
For openPWM1(255) calculation:
(period+1)*4*Tosc*TMR2 pre scale
(255+1) * 4 * 1µs * 16 = 0.016s, Frequency = 61Hz

The measure value for robot moving forward:
frequency = 56Hz
mark = 1.4ms
space = 16ms
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What is the value i should use in DCPWM1(???)
i have done some research on the internet lots of them r using 640. How can i calculate this out?

Thanks,
Dan