What audio level should I develop for

I have asked this question on codeproject a while ago, not getting a sensible answer. Let's try a more varied community.

I sometimes have to develop things with audio for desktop applications, and I come across some interesting "bugs" in this. I can't seem to get my audio level at the correct volume.

I do not mean how to edit audio volume in files, nor am I asking a question related to audio in it'self but rather what percentage of audio, given my analog connection to my computer, (no boost/gain added), should I use?

I use 20% as my "standard" level, that's comfortable for Spotify etc.

Is this apropriate, or is there some industry "best practice"-level to develop/test at?