Where do I set volume control on NAD M51 DAC?

It depends on sensitivity of your preamp.
The idea is to fill up the cup so you can bring it to the table without spilling :)

I have a Primaluna integrated with the following input specs: 

Input Sensitivity: 320mV 
Input Impedance 100k Ohm

Does that help at all?  Thanks Czarivey!

Well, 
At 0db your DAC output level is 2V; at -10dB the output level is 200mV respectively. 
By supplying 200mV instead of 2V you simply void overloading that causes distortion and significant loss of tonal balance and dynamic headroom.

I don't think those figures are correct as the NAD has a different stepping than one might assume. I know the actual top output is 2.375V SE and double that for balanced. -10db is just ten steps from 0db and the DAC can go to -55db before actually truncating a 24-bit file. While I can't say the exact voltage, I am sure the voltage is far closer to 2V at -10db than 200mv. This volume control is from a digital sense 210db in range.

Between the technical measurements and number of user indicated preference, the range that most prefer is between -6db to -10db. If you have a good preamp setup, it won't present any issues. 

I have a C510 which is supposed to be the same circuitry inside. Direct to amplifier it could be harsh & analytical at higher volumes; dull & muted at lower volumes.  Adding a (tube) preamp helped immensely; however as it doesn't have a remote, I find myself using the Nad's rather frequently.  I can go from about -3db to about -20db before noticing any difference and getting up to change the pre's volume manually. I'm sure much of this is system dependent.