R134a has a very tight window of tolerance, often requiring the technician to be within just 1oz of the OEM specified capacity. Thus, a complete evacuation recovery and recharge is recommended.
The most accurate way of charging is to simply use a charging station if available. Charging with 12oz cans is not recommended as the results are often inaccurate. Charging with a 30lb cylinder is a viable option, using an electronic scale to weigh the refrigerant.
NOTE: If charging in liquid form, never exceed 50PSI suction pressure as this may damage the compressor immediately.