Business Portion Only - Asset Sale Price

Hello.  I have seen some responses on this subject but nothing that provides any detailed help for someone who has rented a vacation unit for summers and winters, and uses the property personally for one month in the spring and one month in the fall.  I am assuming that the "Business Portion" would be the ratio of the rental days in summer/winter divided by the days of total use (ie. business and personal).  Is this correct?  It makes a drastic difference on the capital gain amount to be able to reduce the asset sale price through this process.  Thank you.