I simply you a trimmer as one part of that voltage divider. I trim it just by listening to the sound. Therefore I play a low volume audio part and adjust the trimmer to minimum audible noise. Of course, that DAC never reaches true 16 bit, but there is still a big audible improvement over 8 bit, that is especially audible on lower volume parts or if something fades out. Remember: It is made with simplicity in mind - one resistor and one pot and that's it.