Actually I do have them set up to show the fuel flow rate. So I know what its using as I go.
I just use the gauges to glance at when I cant do the maths in my head, or if I'm messing about and not at a steady pace.
Annoyingly cant seem to get the Garmin to show the range left, despite it knowing the size of the tanks and the rate being used.
When cruising I'm using 1.2 litres for one Nautical Mile - so I should get a maximum of 158 NM till I run out.
Italy is only 116 NM away - I should be fine then