Hello vpo-organist,
For the quality of audio recording specifically, all that really matters is that you have 32-bit selected for the audio recording format on Hauptwerk's 'General settings | General preferences' screen, since the audio will then be recorded in 32-bit floating-point format, as Hauptwerk's audio engine uses, and in floating-point formats the quality won't be affected significantly by any level settings as long as the level of the recording isn't so high that clipping occurs.
In general, it's good practice to record audio at the highest level without clipping ever occurring, but that doesn't really matter if you're recording in 32-bit floating point format (as opposed to an integer format) and as long as you keep the audio file in a floating-point format (e.g. if you do any subsequent processing or format conversions in your audio editor).
However, if needed (to avoid clipping, or if you plan to convert the audio file later to an integer or compressed format), or if desired (e.g. so that your audio files end up at a convenient level), you can adjust the level of the recording(s) via the 'Audio recorder: Level adjust' setting for the relevant mixer bus(es) on Hauptwerk's 'General settings | Audio mixer' screen.
Aside from recordings, for background on Hauptwerk's audio level settings, see also this topic:
viewtopic.php?f=4&t=19235&p=144536#p144531