Your formula does this:

503-483=20
20*8000/65536=2
2>>4=0

It works OK.

I think the formula is wrong.

Your ADC has 12bit resolution, so its output is 0-4095.

But your sensor has an offset of half the power supply since it measures AC and DC currents.

Question: Are measuring ONLY DC currents? If yes then do these:

Scale down the ADC: adc=adc>>3 'From 0-4095 to 0-511

Then subtract the offset that is in the middle our new scale: adc=adc-256

Now it should be OK.

Ioannis