Yep, wait till the fuel light comes on & then brim it & work it out from there. The fuel guages have a mind of their own, I thought the same thing in mine (Shitroen ZX Volcane TD) but I'm averaging about 48mpg with a mix of dual carriageway/town work & on a motorway run at 80-ish I get about 55mpg

BTW, the ZX is about 50kg lighter than the 306 so it might not be quite as good as that but very close if it's well maintained