Don't prorate the Use. Annualize it.

Example: 300 SF in use; 3 months of use

300 square feet multiplied by three months equal to 900 square feet total used that year. This is then divided by 12 months in the tax year for an average annual use of 75 square feet, and the total annual use of 75 square feet would then be multiplied by the $5 allowance for a $375 deduction.

Double-check your work:

300 sf * $5 per sf = $1500, and they used it .25 of a year = $375