Considering where the tank is and the difference between what you can get in and the stated capacity, I think they are working on a very liberal co-efficient of expansion.  my PC is slightly to the right of Genghis
2012 HSE SD4 In Orkney Grey now gone, best car ever.

Just filled up with 8 miles showing as left on the trip computer - got 65.76 litres in. Just checked records for my old '07 plate Freelander and the most I ever got in that was 67.14. Assuming no changes to the tank size (and I don't think there have been), I can't see any difference in capacity... looks like I could go to a least '0 to go' and still have some fuel left. Previous car didn't have a trip computer so have no idea about the max I got in and what any computer may have said.

Like the phone app suggested previously 'fuel friend lite' and have now added all my previous fills. 2016 BMW X3 30d SE in red
MY2013 SD4 XS Firenze Red, 19" 10 spoke contrast paint wheels, Signature Strips, Grand Black Lacquer trim, Cold Climate Pack - gone!
2007 TD4 S Stornoway Grey - gone!

Fuel tank in diesel versions has always been 68 litre capacity, fuel tank in petrol versions is 70 litre capacity.

I think the key word is 'useable', the tanks are probably the same size and obviously more than 68L my PC is slightly to the right of Genghis
2012 HSE SD4 In Orkney Grey now gone, best car ever.