In Rev. Proc. 2013-13, the IRS provides this guidance for a qualified business use of a home for a portion of the taxable year.

Step 1. Add 300 square feet for each month of business use.  (If business use area is less than 300 sq. ft., use that lower area.)

Step 2. Divide the sum from Step 1 by 12.

Step 3. Multiply the quotient from Step 2 by $5.

In Lacerte, go to Screen – Business Use of Home. In the Field, Business use area, enter the figure from Step 2, and in the Field 2=elect to use simplified method, enter 2.