I am having trouble working out the math to transform the screen coordinates to the Grid coordinates.

The code below is how far I have got but it is totally wrong any help or resources to fix this issue would be great, had a complete mind block with this for some reason.

    private Point ScreenToIso(int mouseX, int mouseY)
    {
        int offsetX = WorldBuilder.STARTX;
        int offsetY = WorldBuilder.STARTY;

        Vector2 startV = new Vector2(offsetX, offsetY);

        int mapX = offsetX - mouseX;
        int mapY = offsetY - mouseY + (WorldBuilder.tileHeight / 2);

        mapY = -1 * (mapY / WorldBuilder.tileHeight);
        mapX = (mapX / WorldBuilder.tileHeight) + mapY;

        return new Point(mapX, mapY);
    }