The amount indicated on the back was 6.5 lbs but if R134 is used it is 70-80% of this amount. Yes I have found slightly less was also able to be used as before it was determined actually how much was required in the early nineties when it came out and people assumed you charged an R12 system with 134A the same which was not the case because if this was done IE charging till the glass was clear then you were overcharged. Quite often when the happy amount of 134 wasn't known I had to use a thermometer on the evaporator discharge with the engine at fast idle or if it was off road equipment high idle with the blowers on max and slowly charge until the temperature quit dropping which would indicate maximum efficiency and of course if one was "day dreaming" or neglected to shut off the refrigerant entering the system, the temperature would start to rise. This is how I discovered that some systems would work great with as low as 70% 134vs 100%R12. Sorry for the slight off topic re. 134vsR12 but typically the R12 was until the sight glass was clear and then an additional 1/2 to 1 lb of Freon depending on how large the receiver drier/accumulator is and in the case of the Mark II this drier is massive and is located under the passenger side floorboards. It is massive unless a smaller universal fit or aftermarket drier was installed. The sight glass being covered with a film is not the best indicating a bit of a dirty system but perhaps a very bright light will let you see. Going by pressure also works in a pinch and this is where a pressure chart or the indications on some guages are used IE the higher the outside temperature the higher the pressures of both the low and high side. Also a restricted condenser located infront of the radiator or a restricted radiator blocked with insects can cause the pressures to be too high with poor cooling.
And by the way DID YOU CHECK THE OIL LEVEL IN THE COMPRESSOR THROUGH THE SITEGLASS?? This is located on the bottom side of the compressor and when not running you should see oil in the crankcase of the compressor. A very common mistake which was done on my car when it was recharged with insufficient oil and the damn compressor burned out before I bought mine and try to find parts for an HH Techumseh because I do believe the last year they were manufactured was 1957 and parts are nearly extinct resulting in compressors being parted out!! Also another thing I forgot to mention is the 1957 didn't have this oil level site glass unless an earlier one was installed or a later one installed in a 56. Typically the oil circulates with the freon and if you have a leak the oil escapes with the freon and areas where there are leaks quite often have signs of oil such as fitting/hose connections and the front seal of the compressor. As for the cloudy or dirty sight glass and the oil level, if in doubt use a recovery unit and recover all of the valuable R12 and clean the rear sight glass and check the compressor and if you go this rought drain the oil in the compressor and refill with fresh oil. Consider getting your drier reconditioned as well where they cut it open and replace the moisture absorbing desicant because other than dirt and lack of lubrication, moisture is an enemy of an AC system because water combined with R12 creates an acid not to mention freezing and restricting the Thermal Expansion Valve. BTW a restricted or defective expansion valve will give you low to negative pressure on the low side and poor cooling showing a clear sight glass but low pressures typically low to negative on the low and lower pressures on the high. This restriction can be the result of the desicant breaking down or the small bag containing it rupturing and the desicant circulating through the system and ending up in the expansion valve. A lot of times a small screen was inserted in this valve to catch contamination and would require cleaning. If you choose not to recondition the drier a smaller aftermarket one can be installed but of course depending on how small or large it is will create a reduction in the amount of the R12. Slide under the car and check. If it is the large OEM tank underneath throw in another 2 to 2 1/2 cans to give you 6 to 6 1/2 lbs. 4 lbs is not enough for such a large system.