One can is not enough. Did you replace the oil in the system too? Did you evacuate the system? This is not an easy task and there are a lot of steps!!

When the Ac is turned on the extra cooling fan should come on ,if it doesnt then this is your problem as when moving the air flow is cooling the front rad which contains the hot gas ,now the Ac unit any vehicle is weighed in by a special machine and you sticking in a can of gas which is illegal to sell here in europe ,i think your AC should hold around 700 grms of gas ,look under the bonnet ,forget the pressure readings this is weekend boy racer stuff you shouldnt have any pressure on the return to the Ac pump but a suction .If you dont understand any of this then take car to a specialist garage to have the AC regassed on a machine