Do you have a link?  I'm not finding it via Google.

I strongly encourage you to post a question about this on other tax boards (such as TaxProTalk) and see what other tax professionals tell you. You could say something like this:

Client has a mix-use mortgage for $100,000.  $70,000 was for purchasing the home, and $30,000 was for paying off credit cards.  Total interest for the year was $10,000.  How much, if any, of the mortgage interest is deductible on Schedule A?