Can I take a percentage of my mortgage principal and interest for home office use as an expense against my gains from rental property income?

[JoannaB2]

Yes, you may take Home Office deduction if you used the particular space of your home exclusively for the rental property.  You can either use Simplified method up to 300 square ft. ($5/sq. feet) or using the actual expenses on portion of mortgage or rent, utilities, and repairs. The form 8829 is not listed on Schedule E. You may enter the deduction on line 19 "Other" on Schedule E.  

Under the passive activity rules you can deduct up to $25,000 in passive losses against your ordinary income (W-2 wages) if your modified adjusted gross income (MAGI) is $100,000 or less