You can get close, but you will never know for sure.
In the Az Desert the refrigerant expands and contracts so much that you can be fine one day (nice and cool) and not have enough the next day (sweating your butt off). IIRC the whole system holds just shy of 5 lbs of r134a... like 4 and 1/4 pounds or so... the sticker under the hood can tell you exactly though, and I am not going to get off my butt and walk out there to look at mine.
If you go the individual can route don't get the stop leak as all that does is plug stuff up...