Are you trying to cool the house or heat it? Is this a txv or piston system? What is the indoor wetbulb temperature? There should be a chart on the outdoor units back cover. What are the model and serial numbers of both indoor and outdoor units?
did you look at the superheat chart under the cover
on reem its very easy look at the outdoor temp the suction line temp and psi and just cross the chart if temp on suction line is to low overcharged is temp to hi under charged
beer can cold dosent count
Since you have an outdoor ambient of 68 degrees I would guess that this is a commercial application and you are calling for cooling, because I am assuming that by discharge temperature that you mean supply air temperature of 55 degrees and return air temperature of 78 degrees 10 degrees warmer than outside. If this is the case than you have a 23 degree temperature drop across the coil. This usually means a dirty filter, blower wheel , too low blower speed or coil icing up. If I am incorrect and you are in heating it is very difficult to charge a heatpump while in heating keep in mind that your suction line becomes a hot gas line and your liquid line is still your liquid line only in reverse direction. If you are in cooling you might find around a 180 lb head and if the air off your coil is 55 you should have a 40 degree evaporator which is 68 lbs. On this cold of a day you could expect a low suction as low as 58 60 lbs except your return air is warm which will bring it up. If you are in heating just write back and we can still help. Thanx frank