Before each measurement the measuring cell is rinsed with 10 ml sample water. So the total amount per measurement is sample volume + 10ml.
To the waste water, the dosed reagent from the rinsing process and the measurement is added. Further deviations are caused by inaccurate calibration and sample tube length.

The doser can dose over several feet. How far exactly also depends on the height. It would be best if you tried it out.