The easiest way to figure out your water use would be to first find a container of a known volume that fits in the sink, turn on the water taps to the same flow rate you use when you brush, and time how long the container takes to fill up. Divide the volume by the time to get a flow rate such as liters/minute. If you do this several times and average the results, you will get a more reliable flow rate.
Then all you need to do to figure out your water usage is time how long you have the water running while you brush your teeth, multiply that time by the rate you calculated in step 1, and that will give you your usage. Again, doing this several times and averaging the information would make the data better.