In general it is best to keep the signal as hot as possible (i.e. 100%) until the final volume step. The reason is that you may have a noise floor from the computer and you may get additional noise on the cable between the computer and headphone preamp. By running the signal as hot as possible and the reduce the volume as last step, you will also reduce the volume of that noise, hence get a cleaner sound.
There are a few things to keep in mind though:
- some software start to introduce compression/clipping at high volume settings
- the output amplifiers in the computer sound adapter output may start clipping/distorting/popping with very hot/loud mastered music.
- the headphone preamp might not like the input that hot - i.e. it will start distorting/clipping
The only way to find out is to experiment a little and listen. You might go for something like 90-95% from the computer (and possibly also the software you play from). If it doesn't make a positive audible difference, just go for 100%