I'm not convinced it should be 75/25.

The principal repayment rules for mixed-use loans can be fairly complex.  There are ordering rules in 1.163-8T(d):

https://www.law.cornell.edu/cfr/text/26/1.163-8T#d_1

This gets pretty complicated when you have the 8T rules interacting with the 10T rules for qualified residence interest.

At this point, extend.  Unless you have nothing else to do for the next 6 days, you're not going to be able to get up to speed on this stuff in time to get it right.

Rick