If the room is solely used as a home office for work related purposes and you want to claim rent, it would be calculated using the square feet. So it would be the square feet of the room divided by the total square feet of the house.
You would also be able to claim electricity, rates etc.
another way is the ato rate which is number of hours per week x number of weeks x the ato rate of 45cents.
Whichever gives you a greater deduction I would elect.