We have no choice other than to declare an emergency whenever we believe we will be landing with less than final reserve fuel. The timing of that can vary, depending on circumstances but often that happens when you are being dicked about by ATC who won't give realistic information or specific times. But as this call depends on your actual fuel state, it doesn't matter what fuel you started with. So don't let the anti-minimum brigade tell you otherwise. What a Mayday call does is grab ATC's full attention and as a result (normally), give you real numbers you can work with - even if it is for a diversion. As for the reporting, does it make a better story if you land with 23 minutes or 19 minutes remaining fuel? And his would they find this out? Not from me they wouldn't. Or is it the word "Emergency" which allows them to use their aviation terms such as panic, scream, air-pocket, near-miss etc.?
PM