So, how do we go about ascertaining the correct numbers?
If you don't want to accept the ATSB's methodology then you have to come up with your own. R Davies figures were based on the information provided to him by whoever commissioned him so his figures can only be considered "correct" in that context. The ATSB sets out the different methods that Pelair used to determine fuel. Either the fuel/hour figures or the 23lb/min method. I am assuming that the fuel figure quoted at ToD was derived off the FDR. For me the bigger issue is what was done with that fuel once the PIC realised that the weather had deteriorated and they couldn't divert.