Polax
My understanding is that a Mayday call is made when you have a fixed amount of fuel remaining as defined in the Company operating manual e.g. 2000kg's.
No, no and no again. I hope you are not a flier.
Firstly, the minimum fuel amount depends on your weight, which can make a large difference.
Secondly, the regulations state you must be at the final approach fix (FAF) with this minimum fuel. So you have to calculate (guess) you much fuel you are going to use between now and the FAF. So yes, you give the emergency call before you get to minimum fuel.
Actually, if I was at 1,600 kg, and the aircraft minima was 1,200 kg, but I knew it would take 600 kg holding before getting to the FAF, I would personally give a PAN rather than a mayday - and advise ATC that I only have 10 mins holding remaining, rather than the 20 mins they were expecting. That might help ATC in making you number 2 or 3, rather than immediately making you number 1.
More information is better, in this case. A simple Mayday does not confer your exact predicament to ATC, and may put you in front of someone who actually has less fuel than you do (but was not expecting to be shoved backwards in the queue).
Ryanair pilots please note, I have done this myself on three occasions - calculated precisely the exact fuel to the FAF (which was just enough) and thus not declared an emergency, and landed with the required 150 kg below final fuel (burn from FAF to landing). However, if someone had barged in front of me with a Mayday call, who had more fuel than I had, I would not have been impressed.