Car a/c question

Houston TX

Description

My a/c wasn't out but it felt low enough to cause a mild sweat. It was going from sorta cold to non existent cold. So I went to auto zone got 134a refrigerant and before I was told to hold can upside down, and shake as I held down trigger. Well it has a guage on it and I was looking at it backwards upsidedown and when I pressed it I thought I was reading it as low but infact when I finished dumping the can it was at the middle of the yellow and red line holding 60 the entire time. Was it actually full and I just added unnecessary can? The AC isn't cold still and I heard you could add another can. I tried to talk to AutoZone and they kinda brushed me off and said it'll be fine just drive around. Well I drove around for 15 mins it's not cold but also not boiling in car. Does anyone have any experience they can add? It was a 12oz can.