Graham ... you must have a slighter lighter wrist than me
I use RON 98 and usually get a tad under 19 kms / litre for general slab running (I prefer kms / litre rather than litres / 100 km
In essence the fuel gauge is broken down into 1/6 capacity graduations (of course the first is 2 x 1/6 = 1/3 ... Hmmm, yes I was good at maths
I generally count of getting approximately 70 kms for each mark ie (very roughly) 420 kms per tank ...
Hope this helps with the mental monitoring ...