Okay, I think I have a better understanding of what you're asking. Your assumption that the acquisition debt is divided between the primary residence portion of the home (75%) and the part of the home use for business (or for passive activity which produces income) is not correct. The acquisition debt is for the home in its entirety regardless of how the home is divided up and used. Do not think of it as splitting the mortgage acquisition debt between the primary residence and business. There is no paying down one side other the other. It's one mortgage, one debt. Instead, split the ongoing costs of operating the home, such as mortgage interest, property taxes, and utilities between the residence and business. See Pub 587.
