12L (presuming all is useable) might only equate to 18 mins at 75% but at 45% it's significantly more than that. As a minimum, in the US for example, VFR by day requires a 30 min reserve (45 min at night), Oz requires 45 min day or night.
Without the POH I'm only surmising but 12 L useable would be pretty close to a required reserve at 45%. If there are a reasonable number of airfields in the last few enroute sectors *and* the pilot has a good record of his past achieved fuel consumption in that aeroplane then its not such a big deal.
No argument about the benefit of fuel totalisers though! The aircraft I'm on has those and the owner has just installed a JPI EDM-760 on my recommendation. It's just sooooo easy and comforting to be able to run lean-of-peak for excellent economy AND know exactly what's happening to the fuel flow, remaining endurance and what each cylinder is doing. As far as I'm concerned a totaliser & EDM should be mandatory.