It is quite OK to fill up the car within 8-10 km from drop off. If uncertain add an extra litre.
I had the experience last year of being charged for only having a 7/8 full tank by a major rental company at BNE. I filled the car at Nundah, 9 kilometres from Domestic Terminal, to get Qantas ff points from the Caltex Woolworths on Sandgate Road. I even topped it up to the next dollar. I did not know about it until I received the receipt by email.
A polite email stating the facts and reminding the company that I am a preferred card holder and a frequent customer saw the charge quickly reversed.