Doesn't seem right that I would be deducting only 25% or 75% of the mortage interest, tax, etc.
Agree. TurboTax is calculating the Home Office deduction incorrectly when you simply move from one home office to another and don't use them both at the same time. See https://ttlc.intuit.com/community/tax-credits-deductions/discussion/re-warning-turbotax-2019-incorre... for details.
