Prop. Regs. Sec. 1.280A-3(d)(3) states that the same total-fair-rental-days-total-days-used formula applies to allocate the portion “of any item” to rental use.
Turbotax uses the IRS methods.[edited a miscopy]
If you’re looking to claim the most tax breaks possible, you need to carefully track the amount of time you and your family spend at your vacation home.
If you personally use your property and sometimes rent it to others, special rules apply. You must divide your expenses between the rental use and the personal use. The number of days used for each purpose determines how to divide your costs.
Renting Your Vacation Home
https://www.irs.gov/uac/newsroom/renting-your-vacation-home-1
To figure your deductible rental expenses and any carryover to next year, use Worksheet 5-1.
This worksheet will give you the basis for what Turbotax calculates behind the scene.
Personal Use of Dwelling Unit (Including Vacation Home)
https://www.irs.gov/publications/p527/ch05.html
