I just know how much water each of my cups/pots holds. I usually fill them and pour the water into a measuring cup to determine the volume of a full pot when I buy it. I just eyeball it if I'm making, say, a half-pot.
This works for me because I have two pots, one mug, and my cups are effectively the same size. If I had enough that it was confusing, I would probably tape a chart to the inside of my tea cabinet to remember.