I guess this is my cue.
For the situation you mentioned above, you would use 300 for days rented and 0 for personal. The IRS doesn't count vacancy days as part of the equation.
Here's a clarifying snippet from the instructions of page for Schedule 1040SE: "For example, you used your property for personal use for 7 days and rented it for 63 days. In most cases, 10% (7/70) of your expenses are not rental expenses and cannot be deducted on Schedule E."
http://www.irs.gov/pub/irs-pdf/i1040se.pdf pg. E-5
As an interesting aside, my Tax prof actually filed a brief in an appeals case arguing that for interest and depreciation, 365 days should be used because those factors are dependent on time, not occupancy. His position was adopted by the 9th Circuit Court of Appeals, so if you live within their jurisdiction, the ruling can be used to get a little bit more back from the IRS if you have a 2nd home that you used for personal use and rented and you are not otherwise able to capture all of your depreciation for tax purposes. Let me know if this applies to you and we can discuss the details.
Since you haven't paid me for this service, I am not YOUR accountant and this is merely an example for reference purposes. You rely on this information at your own risk and by using it agree to limit any legal damages to the amount paid for services (that would be $0). Have a nice day.