To Answer the Question
A very good approximation of the new CP fuel required is found by taking a ration ratio of the new CFP total fuel/Old CFP total fuel and multiply this by the old CP fuel. This will be within a couple hundred kilos. Use the same mandatory if you had it as the change in it will be very small. You can use the same ratio to find what the fuel required at each point is. We used to use this all the time before the days of the FMC.
You can always then ask for a new cfp enroute to backup your math.
(some w@nker will probably point out that a better ratio can be found by leaving the contingency fuel out of the equation, and they would be correct, but the difference is negligible)
Last edited by bellcrank88; 8th May 2013 at 15:08.