I am trying to get the proper charge in a Thermo King refrigeration unit. It calls for r502, and I am using Enviro -safe's 502a

Here is what is happening now, box temp of 90 degrees, will not pull down, at least by much. Suction pressure of 25,high side 200. evaporator temp just after the expansion valve 35f, temp leaving the evaporator 80f. Turned off unit and only about 1/3 of the evaporator is cool. So it appears that it is starved for refrigerant? Sight glass does not show the ball floating in fluid. Again seems to not be enough refriderant. However I am at the limit that the manual says for suction pressure. So two questions, Because I am using a propane/ethane based alternative are my pressures going to be differant(I understand it high will be lower)so I should really be filling more? Or because this is a refrigeration unit it will not do well until it does get down to a lower temperature?