Basically what you have stated is true but doesn't apply to you if you have a new mortgage on a home that has 25% business use. All of that information is when the loan contains different types of debt. In the case of a new mortgage you have only one type of debt: acquisition debt. All of the mortgage was use to acquire the home only, not for the business. This is fundamental for you to understand.
However, since you are allowed to deduct the ongoing home ownership expenses that apply to the business as a business cost, split the mortgage interest 75-25 and use 75% of the total interest in the calculation under Pub 936 for schedule A and 25% of the mortgage interest deduction against the business income on, I believe, schedule C.
