MAX31856 input voltage range is +/-78.125mV for GAIN=8, the input voltage amplifes 8 and then input to ADC, so the ADC input voltage range is +/-78.125*8, Let's only consider positeve section, the resolution is 78.125*8/2^18(As it 19 bits resolution and with bipolar input), so, the output code at input voltage is Vin should be Vin*8/Resolution(Vin is input at MAX31856, the voltage to ADC is 8*Vin). that is: code=Vin*8*2^18*/(0.078125*8)=Vin*8*2^18*1.6.