I think if it's full to the top it's a total of 1 oz. So from the top (1/2 oz) mark to the top of the cup would be another 1/2 oz. You could probably determine this for sure by filling it carefully with water and then measuring - it should be 1/8 cup.
I have found that if the cup gets more than 1/2 ounce in it, I leak, due to the fact that the cup isn't straight up and down in me, I move around and the cup gets compressed, and a certain volume causes leaks.
may my heart always be open to little birds who are the secrets of living whatever they sing is better than to know - e.e. cummings