I would like to know the effect of the level of
hydration of a person on their ability to focus.

I'm looking for statistics like a X% increase in
dehydration will lead to a Y% decrease in focus
or ability to concentrate.

I've been trained that a 2% level of dehydration within the human body
not only effects your ability to concentrate, but it also starts to
effect your short-term memory as well.  It is recommended that for the
average, not so active individual to drink 1-2L of water a day to stay
hydrated.

Anybody can estimate their hydration level by checking the color of
their urine. If it is dark than you will require more water through
the day, and the color will lighten as your hydration level goes up. 
But you also want to be careful not to flood your system with H2O as
there have been some studies that found that it may force too much
protein from your system causing problems with your liver.