Voltage Reference Resistor Calculation (Shunt)
These are values that come from the datasheets for the respective parts.  The DAC shows the following:



So you need to supply 250uA to the DAC.

Then for the LM4040 to operate correctly it needs a reverse current of:


So at least 74uA for the -5 version.

So the minimum current through the resistor has to be 324uA.  Unless you're counting every uA I'd go up to 500uA at least to be sure you have enough current over input supply tolerance, etc.

With 12V on one side of the resistor and 5V on the other then, the maximum resistor value you can have is (12-5)V/500uA or 14K.  Anything higher may not regulate properly.

This is of course assuming you don't have any other load connected to the reference.