It's refrigerant and not coolant. Coolant goes in the cooling system and refrigerant goes in the a/c system. If you are low on gas the pressures will look kind of normal, the difference is that it will pull the pressure down too low and the cycling switch will kick the compressor off and then back on when it is high enough to engage. So when you hear the compressor cycling on and off it is your indicator that you still have gas in the system just not enough to keep the compressor engaged continuously. This is all providing you are on high fan speed, on low fan speed it will cycle the compressor on and off as well but not as fast and for different reasons. You can try adding half a can or so and see what it does, you apparently have guages so you can monitor you pressures. My advice still stands, take it to someone who does this for a living and have them do a proper evacuate and recharge and then install some UV tracer dye to aid in finding the leak when the refrigerant level gets low enough that you notice it in the outlet duct temperatures. The reality of the situation is that you probably have a very slow leak in the evaporator core and you are not going to find that without the aid of some dye and a UV lamp and glasses. I have done other a/c repairs on 850's but I have probably done 10 or more evaporators for every non-evaporator repair on an 850.

Mark