Refrigeration system charge calculation query.

[refriengr]

I am working on a low temperature refrigeration system (-35 Deg F SST, 8 TR capacity, R404A Dx system, flooded condenser for winter operation).
I am using following estimate of refrigerant charge,

System refrigerant charge = (weight of liquid refrigerant with 50% of evaporator filled with liquid) + (Receiver winter charge)

Is this estimation method correct?
Is there any other better method / approach for calculating the refrigerant charge in a system?

Thanks.