On Jul 15, 10:31 am, "Jon Danniken"
wrote:
When the tank finishes filling, measure the amount of water which
accumulated in the adequately sized container, and add that amount to the
volume of water used by the tank.

This still doesn't completely get you there since the valve is also
spraying water into the tank while the flapper is open and some of
that is going throught the bowl and down the drain. How much depends
on the design and your water pressure. Some fill so slowly this
probably doesn't matter much.

About the only way to measure it accurately would be to either remove
the toilet, set it on top of a bucket and flush it (with the lines
still attached) or find another way to capture all the water from the
drain pipe.