Adding the tolerance control to the dropped zone, this is the option of the droppable jquery ui section:

Specifies which mode to use for testing whether a draggable is hovering over a droppable. Possible values:

    "fit": Draggable overlaps the droppable entirely.
    "intersect": Draggable overlaps the droppable at least 50% in both directions.
    "pointer": Mouse pointer overlaps the droppable.
    "touch": Draggable overlaps the droppable any amount.

http://api.jqueryui.com/droppable/#option-tolerance