Hi TK,
The AWATT register is directly proportional to the watt measurement with full scale inputs approximately equal to 4862401 (decimal). When calibrating with nominal inputs, you will have the value of the AWATT register that is equal to that power reading and using that you find a constant to convert the register value to Watts, as you have done. From then on that constant value can be used to convert the register value. There is no exact formula because it could depend on small part variations, calibration of the part is required to reach the specified accuracy. The calibration procedure is outlined in detail in the Calibration Application Note
Regards,
Dlath.