For future reference, since this was frustrating to calculate. Here is the method for all to use. it calculates the grid square that your mouse is over based on screen size and properly accounts for the Zoom level as well. using the center of the screen as a starting point.
local sw = (128 / getCore():getZoom(0));
local sh = (64 / getCore():getZoom(0));
local mapx = getPlayer():getX();
local mapy = getPlayer():getY();
local mousex = ( (getMouseX() - (getCore():getScreenWidth() / 2)) ) ;
local mousey = ( (getMouseY() - (getCore():getScreenHeight() / 2)) ) ;
local sx = mapx + (mousex / (sw/2) + mousey / (sh/2)) /2;
local sy = mapy + (mousey / (sh/2) -(mousex / (sw/2))) /2;
local TheSquareYourMouseIsOn = getCell():getGridSquare(sx,sy,getPlayer():getZ());