that high side gauge is virtually useless. It dos not show the amount of co2 in the cylinder like your gas gauge in your car does. It only shows pressure. The pressure inside of the cylinder will remain the same right up until it is empty. Pressure inside of a co2 cylinder is temperature dependent. It will show a lower pressure in a colder environment like in your fridge than it would at room temp, but you will still have the same amount of co2. Five pounds of co2 will always be five pounds of co2, regardless of temp, it will just have a different pressure.
edit- the only way to know how much co2 you have left in the cylinder is to weigh it. Take the regulator off of the cylinder. Look at the side of the cylinder towards the top for a stamp that says "TW10" or something along those lines. The TW stands for tare weight. That is the weight of the cylinder itself when it is empty with no regulator or anything else on it. So if your TW is 10 and you place it on the scale and it reads 14.5, then you have 4.5 pounds of co2 left.