Home office opinions please

The space is supposed to be used exclusively for business, rather than just used for business sometimes.  I think you either have someone that doesn't understand that concept or is really bad at math.  Me thinks 75% is probably a wee bit high.

Can you pull the address up on Zillow and see what the total square footage of the entire home is?

Then just ask for the measurements of the studio and storage areas. Math will do the rest.

I agree with the prior comments.  It seems that 75% is extremely high, but it is hypothetically possible if it is a large apartment.  If you doubt their percentage, ask for measurements of each space and emphasize asking which spaces are used EXCLUSIVELY for business.

He's probably not making any money with music anyway, so it doesn't matter.  But what did he tell you when you asked him the total square footage of the apartment, and the total square footage of the rooms in question?

You did ask him that, right?  

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.