The business use % for your car expenses should all be the same amount. So if you are claiming 80% of your petrol as a business expense, you should be claiming 80% of any other car related expenses also (things like car insurance & car maintenance.
To work out your business use % for car expenses, you need to keep a logbook. Read our post here about how to keep a log book.
If you have not finished a log book yet, you can estimate the business use % for this BAS. You will then need to continue keeping the logbook until you finish the 12 week period. You can then adjust your BAS at the end of the year if the business use % is different to what you estimated.