My spouse plays upright bass, does arrangements and recordings and performs (and yes, makes money at it). We decided to use the simplified method which has a max. Here's the way we judged: can you shut the door and walk away and not need that space when not working on music? We could. As for the music and storage being 75%, he might be looking at floor space, not dedicated space. "Heck, this place is pretty much stuffed with equipment." It's not just making coffee with a few flutes on the shelf above you. 

Obviously, performances are elsewhere, and the recording space likely isn't a post-production studio (does he have a sound booth?), but most likely a desk space with electronics equipment (stacks and racks) for transcription, laying down rehearsal tracks and arranging parts for review. The footprint of this can be pretty small, even with a drum kit and piano. And the computer set up can be used for all sorts of things, unless it is a specialized setup, which ours is.