If I share my home with a renter (typically short-term) but there are no explicitly marked spaces for exclusive use, how do I calculate expenses, depreciation?
Generally, there "has" to be at least one room that is for the exclusive use of the renter. That would be the bedroom. So at an absolute minimum that room is for the sole and exclusive use of the renter. If a two bath house where there is a bath that can only be accessed from that bedroom, then that bath is also for the exclusive use of the renter. So I'm not seeing how you can't have a clearly demarcated area for the renter, unless there's something about this arrangement you're leaving out.
I understand but this is a slightly unusual situation since that renter is also a friend/relative. The bath is shared. Also that person sleeps in the living room or sometimes a bedroom if there aren't other friends over. The lease does not say anything specific about exclusive use of any portion of the house but that certain portions of the house are shared.
If a relative, then I would just consider it a cost sharing arrangement and leave well enough alone. You're not making a profit and never will, since what you're being paid most likely doesn't even cover the mortgage interest.
Otherwise, for taxes you "HAVE" to designate a percentage of the floor space as exclusive use. Weather it actually is or not, is irrelevant. But I still recommend "don't go there" since it's a relative. You're just creating more unnecessary paperwork that could very well start screaming "HEY! IRS! AUDIT ME NOW! PLEASE! HURRY! QUICK! FAST!"