I have operated extensively from the USA where only Jet A is available and have always had a fuel freeze analysis done during refuelling. Three samples are taken, one at the beginning, one mid way through and one at the end of the refuel process and all three tested. The one giving the worse case senario is then given as the fuel freeze for that load. Experience has shown that JetA is ok down to -46.5C so -40C is a very conservative figure.