OK. Here goes. I'll assume that you have either seen a SWORD plan and/or have one at hand.PLAN REM= Reserves + Nominated Diversion + Contingency. Contingency is assumed to have been burnt with respect to burn calculations for the diversion, despite that fact that all of it MAY remain upon reaching the hold.This would therefore provide additional holding capablity to that at the planning stage.
Assuming therefore that Contingency has been burnt en-route, one will arrive at the hold with TOT RES. This is Diversion ( GO- around at destination fron the DA/DH to Touchdown at nominated alternate using the aircraft weight as described above) plus Reserve ( 30 minutes of holding at 1500ft clean at an AVERAGE weight).
Therefore if one expects, during fuel planning en-route or at the hold that the aircraft MAY land with less than RES, a PAN call should be made. If there is no doubt that a landing with less than RES will occur then it is Mandated (under a Jar OPs rule somewhere!) that the aircraft Commander must declare a MAYDAY.
Ouch! You asked for it! It is an answer, BUT not perhaps the best one that could have been provided. When Captain Airclues comes along again he can give me a mark out of 10 and perhaps add some JAA quotes! Whatever you do tonight; DO NOT spend it pondering the joys off FUEL POLICY!!
Grazzler