In my experience you always charge on the low side port if you're just 'topping off'. Having said that, cars these days are very very pressure dependent. Some vehicles take as little as 1lb of R134a for a full charge. If you overcharge the system, you can blow out the seal in the front of your compressor or cause over a thousand dollars in damage. I would highly recommend taking it somewhere and having them recharge it for you, they can check for leaks and assure you that your system has a proper charge for less than $100.00.