Typically, whatever percentage of space is exclusive to the renter, is the same percentage of other expenses that can be reported on the SCH E.
So if 30% of your floor space is exclusive to the renter, then you can claim 30% of your rental expenses on the SCH E. There is one caveat though. If you have a landland telephone, the IRS says you can't claim any portion of that cost on your SCH E. (Pub 527 at https://www.irs.gov/pub/irs-pdf/p527.pdf page 16, 2nd column, 2nd paragraph). But any utilities that are exclusively for the tenant (such as a 2nd phone line) are 100% deductible.
Additionally, the shared utility must be available to the tenent for the exclusive use. For example, if you have satellite/Cable TV and there is not a cable drop/satellite box available for the exclusive use of the tenant, then you can't include any portion of that cost on the SCH E.
Now using the percentage of sq ft method is only one of the permissible methods. The other is based on the number of people. See Pub 527 page 16, 2nd column "How to divide Expenses".
Overall, I prefer to chose the method that keeps my depreciation deduction as low as legally possible. You can use SQ FT percentage for some things, and number of people for other things. But whatever method you chose for depreciation, you're stuck with. You can't change your depreciation method once a selection is made.
