The only thing that can be said for pressures are the pressure of the liguid/gas condition at a particular temp.
There are tables that will give this value for different gases. To give an example: say I have a 30lb cylinder with 10 pounds of R12 inside. If you go to the table and look at sea level with a 90 deg day the pressure will be exactly xxx lbs. Over 100. If you have the same cyl with 2 lbs of R12 with the same temp/altitude the pressure will be the same. If one keeps removing and let stabilize the refrigerant, there will be a point at which the liquid boils off maintaining that pressure untill no liquid exists in the container. At this point the pressure starts to drop and continues in proportion to the gas remaining in the system.
So, a proper charged system will have xxxlbs at rest pressures equalized low and high side. A system with half a charge will have exactly the same pressure because at any temp the equilibrium between liquid and gas is a constant.
As to the dynamic situation, your pressures are generally all right. The low side is a little high if you did your performance test properly at 2000 rpms. Your hi side pressures are probably low but have more to do with the heat exchanging properties of the test. Was there a good fan on the condenser? How long had the car ran in one place? The tell tale is of course the sight glass on the R12 system. With your pressures and the visible foam in the glass I would say you could probably use any where from 0.5 to 1.0 lbs. The proper way to know is to recover the refrigerant and weigh out the proper fill.
As another example of the nature of pressures in refrigeration systems. Two cylinders of refrigerant are setting on the floor and are not labeled. If one were to know that one was R12 and the other R134a the chart and a guage and a thermometer would tell us which was which.
Bosch Master, ASE Master, L1
33 years MB technician