If you enter you rent under "Expenses which you incurred for the entire home", then TurboTax will automatically calculate the amount of rent that you can claim, based on the area of the space used for business and how much you work in there. The "expenses which you incurred for office space only" section is for things you pay for that are just for your work area, ex: ink & paper for a printer you only use for work.
You can enter your rent under the "zero rated & expense" column if you didn't pay any sales tax on the rent.
