A/C temp

My car 72 Swinger 318 with factory A/c was converted to 134a by a A/c repair shop at some point. It has a slow leak somewhere in the A/c system. I refilled the system today until it began blowing cool air. The outside temp was 81 and very humid. Inside the car, with the A/C on it was blowing cool at between 55-58 degrees, while idling. It is equip with a two speed blower motor. When driving the car, the A/c doesn't seem to be cool enough, at least by modern car standards. Does that reading at the A/c vents sound like it is cooling properly??

Thanks

Scott