Best way to attenuate R2R DAC output?
The R-2R ladder has a fixed output impedance of R. If you shunt it with a resistor Rx you can attenuate the output equally for all codes. The output impedance will obviously be R||Rx. 

You thus have two degrees of freedom that allow you to set the output impedance and output amplitude independently. 

In this case, if you set R = 353 ohms and Rx = 95.2 ohms you will get the desired characteristics (full scale output voltage of 0.7V with 3.3V supply and 75 ohms output Z).