How to set the resistor value for INA219
Okay, so you have 20A and 2m\$\Omega\$ so the maximum voltage will be 40mV. 

Current LSB according to Eq. 1 is 20A/2^15 = 0.0006103515625A. 

Cal register is trunc(0.04096/(Current_LSB * Rshunt)) = 33554 = 0x8312 

When you read the current register, you multiply that value times 0.0006103515625
to get the current in amperes. 

Similarly, to get the power in watts multiply the value read from the Power register
by 20 * 0.0006103515625 = 0.01220703125.