I normally set mine to 100%. Occasionally, but not often, we'll trim it back. I recently recorded a band and they asked for a patch into Garageband on one of the musician's laptop. The little volume adjustment slider in the laptop software was absolutely miserable, so we fine tuned it using the R09 volume.
Remember that the R09 also does optical out as well, which is a feature that most of the newer recorders have abandoned. I used to run that into H102's and JB3s a lot a few years ago.