How can I calbrate my power meter?
THere are 2 or 3 power phasors computed for each Line voltage and current: 2 lines and 1Delta ,  Assuming you are  not in North America the line phasors angles are at 2phi Vac/120 deg and in North America split 240Vac/180 deg. 


Phasor power angles are straight forward 
PF = |cos φ| = 1000 × P(kW) / (√3 × VL-L(V) × I(A))
Each line may be calibrated for resistive and reactive loads or some matched combo that is -45 deg then compute if any phase error.
P(W) = |S(VA)| × PF = |S(VA)| × |cos φ|
kW = √3 × PF × A × V / 1000  * k (scaling factors)
you already have the RMS sample conversion formula
As far as the ADC is concerned a stable Vref is used for conversion and it may have errors for ; offset and gain and missing codes (monotonicity error) due to digital noise crosstalk on the analog Vref or signal  like 00011111 to 00100001 skipping a step 
good design self-checks calibration using a precision slow ramped test signal to test every code,  linearity, gain and offset error. with correction factors ( sometimes  done yet often ignored )


I used to work for a company that designed wireless 2 way meters for power, gas, water using the ISM band with multi tiered data repeaters at 928MHz in the mid 90's. 1 second response time but scheduled Mux's otherwise.  Headend software integrated into Utilities database.  But technology got sold out to Itron in Seattle. a prime supplier of US power meters after 7 yrs. I was there for the last 4.  Iris Systems Ltd. Winnipeg.

Traceability to NIST standards and Utility Specifications are two topics for further discussion.

Cheers
Tony