That is a very dependent on the outside temp number. There should be a sticker on the radiator support that tells you how much in ounces R234a needs to be in the system. That amount will give you what you need pressurewise and cooling wise assuming your a/c is working correctly.
You have a leak somewhere so in that instance. It is not working correctly and the right way is to evacuate your system and put in the correct amount of refrigerant wit leak detector dye and go from there.
To do a do it yourself. You need a manifold gage set to get both high and low side pressure, an accurate thermometer, measure the outside temp, get humidity level as well and find a temperature/ pressure chart for R134a on the web and pay attention to your gages
Do not just chuck a can in or use only the low side pressure...
AJ