Calculating gain of ICF amplifier
Is this the correct way of approaching this?


In my opinion you might be making things too complicated. You have R3 and R4 that set the voltage at the reference pin so, instead of trying to drive your DAC offset control voltage to the FB pin via \$R_A\$, just drive the reference pin with your DAC output. Use a small value DAC output resistor (maybe 100 ohms) followed by a 10 uF decoupler to stop DAC clock noise then feed the ref pin.

If you want to restrict the offset range control from your DAC to give you less granularity then you can attenuate it.