Temp gun is not the best thing for measuring air temp coming from vent, but a 28 degree temp difference is passable depending upon the humidity level. If the refrigerant (probably R134 not Freon) level was low enough that it would cycle the pressure switch you would not see that big a temperature drop. Quick and dirty check, grab the metal part of the line coming from the evaporator to the accumulator and see it it is cold. If it is, you are at least close to the correct charge. If it is warm, have someone put a gauge set on it and check the pressures.
