any time a fabric isn't square after tearing it to get the straight of grain.1) dampen the fabric with hot/warm water, 2) grasp opposite corners and tug, this straightens the off kelter fabrics. focus on the corners where it isn't the same length.
3) if this doesn't bring it back to a usable size, then once it is straight, cut it in two pieces and run another contrasting piece down the middle that brings it to a size that is large enough for the top. or put a border around the outside edges. Note the middle piece is easier.