Weigh it on a scale? I rented one of those vacuums from autozone. How would I weigh the refrigerant already in the system? And why does every video on the internet use the pressure gauge on the can to fill it?

I am stumped right now. I have a 2015 Nissan Sentra SV. I hooked up gauges to low side and high side. Low side was too high, about 60. High side was correct pressure. I recovered some of the refrigerant to the correct psi based on the ambient temperatures. The air was blowing hot. Then i added it ba...