Keep an eye on the gauge as you fill and you'll get the right amount of refrigerant in. Don't worry if you are off by a few pounds, it's not a big deal. The gauge in the above photo only reads 10 pounds, so this system was very low on refrigerant. Fill it to the proper level and you'll have cold AC once again.
When you're finished filling, don't forget to put the cap back on the low pressure port to keep gunk out. Even if the can is empty, hold on to the pressure gauge because not only can it be used to check your AC system pressure, next time you add refrigerant you will only have to buy the can!