BEagle, hi there.
However, the estimation of fuel required to reach the alternate and carry out an IFR approach is far from simple. Aircraft designers will pretend that low values are sufficient, so that their customers can dispatch with the absolute minimum of fuel to maximise payload within RTOW constraints (or whatever JAR calls it).
Well, I have to disagree on that one, I'm afraid. It's indeed very simple. My company uses LIDO system and it's very accurate. It takes into account the departure from the filed approach RWY at destination, SID, the route, STAR, type of approach, RWY at the alternate. It also take the the appropriate level into account, whereas the FMC doesn't. That's one of the reasons why one is ought to correct the diversion fuel on FMGC INIT B page. The problem with FMGC is the level that FMGC calculates for the diversion.
It's a default level which might significantly diverge from the real one. As an example for a diversion of something like 320 NM the system calculates with the FL 320 way off the reality. In real live one will get FL220 doing the same flight as a normal departure and arrival.
However regardless of whatever FMGC is calculating (mind you it's only advisory) the OFP must reflect an accurate figure representing a legal guide line for the commander.
Cheers.
