Primary residence vs rental allocation

Hi,

 

I have a triplex that I live in, renting the first and second floor while living in the third. As a result Turbotax allocates a % to my personal income taxes and a percentage as an investment.

 

What is the most appropriate way to allocate primary vs rental?

 

Is it by number of bedrooms? Square footage? And how is a private unfinished basement for my own use counted?