As you can see, charging with the can upside down sends liquid refrigerant into the system very quickly and is an easy way to overcharge and damage your a/c system. I'm not an authority on this but I learned from someone who is. What he had me do is hook up the can and hose to the low pressure fitting with the car and compresser off. Then slightly open the end of the hose closest to the can until you get a slight hiss (this bleeds the hose of air). Next put a thermometer into he center vents and start the car and put a/c on high fan speed and recirculate. Start sending R134A into your system with the can right side up (gaseous state). The compresser will pull R134A out of the can as it cycles and this may take some time. Keep and eye on your thermometer to see that your vent temp. is dropping. The best I can get is about 55 degrees when ambient air temp. is 90 degrees. Again, I'm not an authority on this but this method seemed to work well for me. The most important thing is to find out why you need a recharge anyway. Leaks are expensive to fix and hard to justify if you can go a couple of months per charge but if it's a big leak, you're wasting your time. Hope this helps.

Jim