You need all available means to make a solid decision.
LIDO FLTPLNs are calculateted for 4 ALT and with a very favourable RTE. Sometimes allmost DCT. These all 4 might just have become obsolete and you need a new set of Airports. So how do you get a more realistic mileage and fuelprediction? Use your FMS!
If you change the DEST on ACT RTE then there will be an automated message generated to your Dispatch. We had a Tropical Cyclone over Manila and used this tool to try a couple options... After a while we got an ACARS from FRA where the heck are we going to land.;-)
The Fuelpredictions on the B744 are pretty unaccurate towards the end. You can almost every time subtract a 1000kgs from the predicted fuel on arrival. Especially if there is a big difference between calculated and totalizer. So make a correction for that.
RTE2 is a good option, just dont execute it. Try to put next waypoint on LSK 1L and a new DEST. Optimize the CRZ alt with /F150S for example. The problem lies in the details.What Wind will you have? Its pretty tought to figure out what FL you get, and that makes a big difference. So i would go with mileage, divide with a conservative TAS and get flighttime. Multiply with average FF and get Tripfuel. This would be Overhead to Overhead Planning. Pack your personal Extrafuel on top and thats pretty much is it.
The FMS predictions have one great disadvantage. GIGO. In lack of valid input the predictions are useless. They just pretend to be accurate to .1 tonn.
Cheers
L