tm&s - the MAYDAY does not depend on actually being below final reserve, but on the fact that you WILL land below that.
EG FR is 1200kg
I have 1500 in tanks
I need 400kg to fly my alternate = MAYDAY
I have 1700 kg in tanks, normal diversion routing (LGW-STN say,) might be via Woodley which would require 600kg (all figures examples). Then I would PROBABLY declare a PAN. If I did not get (and I'm sure I would!) an expeditious routing I go up to MAYDAY.
That's how I see it. It is fairly simple in my mind and I hope that accords with the ATC view.
Europe: MAYDAY in both cases