How do I calculate the DDS tuning word for a desired output frequency?

The output frequency (Fout) of a DDS is (M(REFCLK))/2N Where: Fout = the output frequency of the DDS, M = the binary tuning word, REFCLK = the internal reference clock frequency (system clock), and N = The length in bits of the phase accumulator. Note: The DDS output frequency of the first Nyquist zone is limited to 50% of the SYSCLK, but is typically limited to 40% due to the sin(x)/x rolloff of the DAC output.