If only one Fuel Penalty Factor (FPF) is applicable:
TRIP FUEL PENALTY = (FOB - EFOB at DEST) x FPF
The FMS fuel predictions must be recomputed to take into account this trip fuel penalty.
This last sentence is a trifle confusing/misleading, the paragraph is an extract from an A320 QRH. Does it mean that the FMS must be reprogrammed to reflect both the chosen diversion airfield as well as the greatly reduced CLB/CRZ/DES speeds and possibly lower CRZ FL? And once done, the revised EFOB is applied to the formula? Or is it the original "clean aircraft" trip fuel profile that is used?