I would keep the screen 2 inches away from the bottom board.  I read a post
where a study was done showing that a 2 inch distance prevented mites from
reentering the hive.

putting a screen mesh onto your existing bottom board will do nothing to
prevent mite infestation.  The idea is to let the mites fall through the
screen onto the bottom board.  I would also recommend having a sticky
material on the bottom board so the mites wont go anywhere.

regards


Rich B