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You have asked what appears off the top to be a simply question.

When you throw in the word "accurately" the answers become multiple and may be considered ball park sized as not complete!

The pressure gauge on your manifold set has a cross (conversion) over temperature scale built on to the face for the most common refrigerants used. There are also charts to be had for those that are not.

The next thing I can tell you is a "rule of thumb" which is not accurate in a real sense.

The basic idea is to run either the condenser or the evaporator temperatures with around 30 degrees differential above or below ambient.
The reason for this is to create an incline for the transfer of heat at a given calculated rate for the sizes of ALL the components in the system.

The low side of your particular systems design is very pressure differentially constrained due to its metering device the orifice tube and the low pressure cutout switch.

To even try to "properly fill" the system requires knowing pressures on both sides and their temperature exchange rates under various conditions. This system is design under laboratory conditions for the best "average" environment. Those laboratory guys have "weighed" their studies and decided the proper refrigerant amount to be used for the most accurate average to prevent under cooling cause by several factors.
One of those would be icing up the evaporator when it is 50 degrees outside!

In this case the air is already crossing over the coils as cold as you would want it to operate at in the summer. Not a good environment to accurate fill a system on pressure alone when its at one end of the scale and know it will work correctly in my opinion.

You really need to pull out any refrigerant into a empty vessel and weigh it. Then replace it and add any difference back to the required weight specification given for your system.

This is how I do it.
I keep an old empty bottle that I pull a vacuum on and set in bucket full of ice.
Its my home made recovery unit.
I have the cars compressor pump itself out to zero and then suck it back in. No guess work with a 30 lb. postal scale from Harbor Freight.

I admit its more of PITA than the old sight glass (that only looked at the high side) systems but that's a different metering and controller system.

If you are only charging your system for defrosting the windshield this winter I would only put enough gas in so that it quits short cycling but still cycles off when you turn the fan speed to its lowest setting!

Properly charge it later by weight for the best heat removal during the summer months.

That is only a rule of thumb recommendation looking out over it (the thumb) into an empty seated ballpark! That's because there are several people that would not tell you that!

Phil