At normal process conditions, the BFW will vaporize into steam. So you should use the same mass flow rate of BFW that you use with steam. You may see an increase in firing due to the energy input for the heat of vaporization.
It is rare to see a plant with both steam and BFW injection at the heater inlet. It is normally either/or. Does your design design have both? I’m guessing the BFW is used only during online spalling per the design?