Home business – tax

I’ve tried a search but can’t find the answer to a tax question.

I know that if I run a business from home, which I intend to do, full time, I can claim tax deductions on the rent on the amount of the percentage size of the room I make the office. My question is….
(and it may be a unanswerable question as the tax department may make decisions on a case by case basis and it will be a matter of pushing will bring unwanted attention to me )..

but anyway, if anyone can help from experience or any accountants..

I want to set up an online busines, so I really space wise only need a desk and a computer and a filing cabinet really, but if I made a rumpus room the office, could I claim this room’s full size as the office as thus claim the full percentage of the room for tax purposes.

Naturally the room wouldn’t be used for any other purpose other than for the business but would have alot of empty space!!

Thanks for any help while I try and work out my move back to Australia.

David