How much should an inactive cat eat?
You’d probably like a table listing the weight of an inactive cat and the amount you should feed them daily to bring their weight down or to stop them putting on weight. However, I have decided that there is a better, more flexible and accurate way.
A person who is asking how much an inactive catch should eat is likely to live with a cat who is overweight (or gaining weight) and they want to manage their cat’s weight. Because every cat is slightly different in weight and size and because there is a range of inactivity levels it is difficult to provide a table of data which covers all circumstances and all individual cats.

Cat body shape guide. Image in public domain.
Monitor weight
I notice that on Amazon you can buy weighing scales for cats which cost between about £20 in the UK (they will be about $20 in America) to about £60 in the UK or $60 in the US. I think these prices are pretty good so I would buy one. I would then weigh my cat and record it.
If she was visually overweight I would give her a little less food than I normally give her and keep on doing that and at the same time measure her weight on a daily basis or perhaps every other day. Write down the details so you can monitor the weight loss accurately.
If there is no weight loss, I would feed her a little bit less till you start seeing a gradual weight loss. When you achieve it you keep on that diet until she hits a weight which you consider to be satisfactory. You can do that visually by looking at your cat from above. You should see a slight waist just in front of the hind legs – see picture:

Correct weight cat. Photo: Warren Photographic with his permission.
Visually assessing cat’s weight through body shape
To be perfectly frank, anybody with a modicum of common sense can tell whether their cat is overweight or not. Just try and be as objective as possible and don’t distort what you see by normalising an overweight cat as a cat of normal weight. This is what can happen. People become subjective and they are unable to really look at their cat critically and decide whether their weight is normal or not. But it should be quite easy to recalibrate your brain and start obeserving objectively!
Change amount of food rather than the calory content of the food
When I’m proposing here is for the cat owner to manage their cat’s weight through the amount of food that they give them. You can buy dry cat food prescription diets (lower calorie) which are designed to bring down the weight of a domestic cat over time. You can try them if you like but I think the simpler method is simply to provide less food rather than food that contains less calories.

Cat weight chart. Image: PoC.
Decent quality wet and dry food
Once your cat is on a gradual weight loss pathway, you can carry on as you are provided the food that you give her is of decent quality. I would not feed my cat with low quality dry cat food. It should be wet cat food with high quality dry for grazing at night perhaps. I think high quality dry cat food is satisfactory but the low quality stuff I think is no good.
Just make sure you feed her a good diet with the highest quality food that you can and in the right quantities as per your weighing regime described above. You can then manage your weight downwards to an acceptable level. And you will decide that level visually if you want to keep the process nice and simple and straightforward.
Starting guide for standard cat
If you want a starting point this picture may help but I have decided that you can do this through weighing your cat and using a visual assessment of weight as mentioned.

Weight of wet cat food daily for a cat of normal activity. Image: PoC.