Like I said before, talk it through with some of the trainers. There's a lot more to this than meets the eye. The OP mentioned crit pts, which could be ETOP points, that can be dealt with by selecting another ERA. Mandatory Fuel, is a bit more complex as you probably don't have scope to select other ERAs. Do you need it once you dispatch anyway???
I agree with Firewall and have experienced the same with a large 0FW drop (5T). Had we used the corrections on the original CFP we would have dispatched with 500kgs less than the new CFP calculated. The corrections assume linear changes and over a small change-weight range are a useable approximation, however over a larger weight change and a long flight these assumptions get stretched.
Again, talk to to an adult, but when carrying Mandatory, I reckon our Fuel Required is calculated on the basis of depressurising at the most critical depress crit.pt, and then we fly at FL140 etc, which means we are so far off Optimum Level (particularly in the 747) for a long period of flight. Put all this together, and I am cautious when carrying Mandatory on ULH, the change between weight and burn is not linear and ratios become unreliable proportionate to weight change.
Don't get me wrong, the corrections are good 99.9%, however on ULH when carrying Mandatory, I am cautious. ETOPs, different game as you can probably pick other ERAs to launch, the use the FMC airborne to finese and monitor.
Clear as mud?