This calculator
http://www.csgnetwork.com/r134apresstempconv.html
will tell you the pressure in your can at ambient temperature. Should have been somewhere around 80 psig at the beginning. That would also have been the pressure in your system before you started the car and turned on the AC.
The can pressure drops as you empty it because of the cooling effect (which you felt if you held the can in your hand). So long as the pressure in the can is higher than the pressure on the low side of the AC system, refrigerant moves from the can to the AC. I usually put the can in a pan of warm water to keep it warm and speed up the process.
The AC pulls a 'vacuum' on the low side of the compressor, between the compressor and the expansion valve. The longer the compressor runs, the lower the pressure on the low side will be. If you put too much refrigerant in the system, the compressor will see lots of high side pressure and will turn off sooner, the low side pressure won't get low enough, and you won't get as much pressure differential across the expansion valve. That means the air won't get as cold. The other extreme is not enough freon, when the high side pressure does not get high enough, the compressor runs a lot but still can't build enough high side pressure to cool the air.
The best way to fill an AC system is to empty it and put in a measured amount. The second best way is to monitor both the low and high side pressures, and compare them to a temperature chart, as you fill the system. The way that most of us fill our systems is to drive until it fails to cool well, then add a can. If it cools OK, we drive a few more years.