I've never been audited but this is only my 3rd time doing taxes.

From my understanding, the formula to use when figuring out the percentage you can claim is... (days of the week you are open/days of the week) x (rooms in your house used/total rooms in your house) x (hours of day you are open/24 hours total).

So to use my home for example it would be.. (5/7) x (5/10) x (9.5/24) = 14%. We had initially calculated it this way and my HR Block accountant (I go to a private one now) said she would be okay with raising it to 20% as that wouldn't raise any red flags. Than when I explained that I only use my basement for daycare she said I didn't need to follow the formula and I would be allowed to claim 50%. I would think that claiming 85% would raise a red flag. Unless I am not understanding your post correctly....which could be very possible! lol

You mention that you don't use 1 bathroom and the family room but what about the bedrooms/master bedroom?