I think a point that needs to be made is if you're flying something older or cheaper (and plenty of fules fancy themselves equipped to fly overwater in R22s with four dials on the panel...), that doesn't have FF gauges, you should ALWAYS, ALWAYS, ALWAYS use the most conservative range and consumption estimates you have. As an example, flying in a 206B in the US, use 30 gph (actual burn is between 27 & 29). As far as groundspeed goes, now that GPS is ubiquitous, there's pretty much no excuse for inaccurate groundspeed. No-wind estimates are an extremely unsafe idea unless you KNOW you have a tailwind component... and even then...
I would never, never EVER go fooling around someplace I couldn't land and get anywhere near a fuel margin if I didn't know my fuel flow, didn't write down the time and quantity figures in case the electronic gizmos went south, and didn't know cold the unusable fuel in my machine in case of any and all fuel or engine-related maladies. To do otherwise is just begging for trouble.
To use a furniture-making analogy, cutting fine dovetails is a futile exercise when your marking and measuring is done with a stick and piece of chalk and your chisel and saw are made out of flint. I believe Nick is advocating a fine marking knife and razor-sharp tools.
Question for Nick... now that all avionics gizmos are getting better and cheaper, what kind of new generation fuel measuring systems exist that provide some fair amount of precision and could be retrofitted into lower-cost machines yet still be cost-efficient (flow and quantity). Many of the systems currently on existing aircraft ain't much better than these:
1st generation "I cain't see how much gas is innit... gimme match..." (this one wasn't very successful...)
2nd generation "Gimme stick..."
3rd generation "Gimme stick with marks on it..."
4th generation "Stick with a float on it sticks out the top..."
5th generation "Toilet float thingie what moves a needle an' mebbe turns on a light"
and so on... into various electronic level sensors, impedance systems, and who knows what else... what's next?