Following  Rev. Proc. 2013-13,

Step 1 - 300 sq. ft. x 3 months = 900.

Step 2 - 900/12=75 sq. ft. You will enter this in the Business use area.

Thus, the deduction will be limited to $375 for 3 months. This equals to one fourth of 300 x 5 =1,500 for 12 months.