Your math sounds about right.
Keep in mind that Koji may not eat well for the first few days after his surgery, so if he goes in carrying an extra pound or so, it may be lost if he loses his appetite.
I am feeding 15 yr old Wendell about 2/3 of a can at each meal (He weighs 27 lbs., but should weigh about 29), so I am trying to put weight on him. He lost 2 lbs. in the past month, (he has become very picky in his old age).