The raven and astro will use 60Lt/h with the raven II it's more like 65Lt/ph
If that light comes on, (and you should only ever see it on the daily) you have sod all time to set down. Like, a couple of minutes if that.
I use a minimum fixed reserve of 40Lt. That 40Lt weighs 29Kg. Makes hardly any difference to ops, but all the difference to health of all involved.
p.s. The only legal requirement for the accuracy of fuel guages is that they read empty in a nil useable fuel situation. Don't trust them. Trust your stick.