904, in Perf Init menu on page 1 below the Aircraft Database information there is a line that says:
CURRENT GS/FF, that means that the current value (whichever may it be) is what the FMS uses to calculate the GS/FF for all the other legs of the FLT PLN.
If you want to change that for each leg you would need to use the alternate PILOT SPD/FF in the computations.
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