It's not a fixed percentage.  It depends on the average balances which will be recomputed each year.  There are also taxpayer friendly ordering rules for repayments, see IRS Pub 936:

https://www.irs.gov/pub/irs-pdf/p936.pdf

There's a section on Mixed-use mortgages starting on page 12 and some examples and worksheets.  Since the "home equity" debt is repaid first, often this is only messy in the year of refinance (or the year after if the refi is late in the year).  Once the loan balance drops below the acquisition debt balance then you're back to 100% deductible.

Rick