Refrigerant capacity

I add some to my Tahoe once a year she takes a can or two maybe at most. The entire AC system is flawed and will need replacing what with the leakage etc. However when she has had enough the gauge tells me that I am getting close to the red line on one of the feed input tubes.

I have no way to use the other output tube and bolt off for recovery of the fluid so I just continue to cram in more now and then as it loses it over time.

Nothing complicated, it is literally a walmart walk about line with shelving and on that shelf is cans with input connections and a gauge on it. It's not the most accurate or best way to do this but it's puts off a future need to burn out the compressor without anything at all.