If you want the full set, you need the coordinates -2,-2 to +2,+2.  I would guess the incoming pixel fragments have screen x, screen y as the inputs, so you'd need to do
inputx = (x-(screenwidth/2)/(screenwidth/4)
inputy = (y-(screenheight/2)/(screenheight/4)
and plug those inputs into the mandelbrot equations.
Jim