Self Employed for 4 months, how do deductions work?

Hi,

I live in Az and was self employed for 4 months working from home.  I heard about the simple $5 per sq ft of used space deduction for the year but to my understanding, that is if someone is self employed for the whole year. 

Since I worked only 4 months, do I just take the $ value of the calculation and divide it by 3?

Thanks