If the dimensions of the shipping box are all 1.5 times those of the gift, then the shipping box is 1.5*1.5*1.5, or 3.375 times larger than the gift. If the shipping box is 1080 cu. in., then the gift is 1080/3.375, or 320 cu.in.. Therefore; the amount of unused space is 1080-320, or 760