I just use the hose and buy the cheap little 12oz cans and fill through the low side port.

The cans with a gauge are kind of pointless because they only tell you low side pressure and really the high side pressure is more important. You'd have to shove alot of freon in the system to max out the low side and do damage.

Adding 8-12oz should be enough to tell you if it works or not. You can keep the little can on the hose if you don't use it all, just spin the needle down untill it bottoms out.