Opamp Noise due to Source Resistance
The wheatstone bridge you are using will probably not have massive resistance value differences under all weight situations so you can use the nominal resistance with hardly any error. 

For an instrumentation amplifier configuration you can ignore the feedback resistors if RG is the dominant lowest resistance (which it usually is to achieve high gain) and then you are left with current noise into your nominal bridge resistance multiplied by voltage gain as the noise produced across the differential output.

If RG is significant compared with Bridge_Resistance then the current noise that produces a voltage noise via RG will also become significant but, this isn't usually the case on high gain systems.