The way to properly charge the system so that you know it has the correct amount of refrigerant in it is to drain it completely, connect a vacuum pump to the system to evacuate it entirely and remove any residual moisture and then recharge the system using an a/c charge scale to measure the exact amount that you put back in. Doing it any other way is a guestimate, surely you can get close but will more than likely be a little over or a little under. To judge how much you put in to simply top it off you will have to go off of the low side pressure which will change with the ambient temps, it will be higher when hotter outside and lower when cooler. You have no access to the high side on that system and if you wanted to get that number it would be necessary to hook up a scan tool and read it off of the the high side pressure sensor. Your other option would be to evacuate the system completely and then recharge it off of your cans, figure out based on the amount that each can holds versus what the system total is just how many cans it will take. You will probably end up at something like 2.5 cans depending upon what size cans you have.

Mark