How do you convert kcal to calories for cat food? How to determine an average feeding amount?

I have a cat that is massively underweight and has kidney cancer, so the more high calorie food he eats the better. I know others have the opposite problem: a healthy cat that needs a diet. I read that the average for a cat is 15-30 cal per pound of body weight, but the cans, dry food, and treats all measure in kcals. I vaguely remember from food chemisty class that 1 kcal of energy = .001 cals, but I read that it's the opposite (1 kcal = 1,000 "label" nutritional calories) and that kcals and calories are the same/interchangeable when it comes to pet food. I am mostly interested in tracking how much my cat eats per day (more is always better for him), but I know the real conversion would help a lot of other people with overweight cats too. Does anyone know the real conversion? Does it vary between brands of food? Does the conversion factor in how much just passes through his system undigested (there are 2 measurements on each can of food)? I will ask my vet to explain the confusing label when they open on Monday, but I think a basic calorie conversion for cat food labels will help everyone, Anyone who can explain the two conflicting kcal counts on the medical k/d food I have in the mean time would be greatly appreciated (his vet seems to dumb things down for me for no reason, but otherwise has taken great care of all of my animals), but really I mainly need the conversion so I can determine how many kcals are in a pet food nutritional label calorie and determine how much my cat is eating (enough to gain weight back, enough to maintain weight, etc.)