How many calories a cat needs depends on their metabolism, and that has to do a lot with whether they are/have been obese, their age, activity, etc. Cats that are older and/or obese have especially low metabolisms, and often do need less than the recommended number of calories. Bandit actually needs a little less the calorie formula says--to maintain him at 12-12.5 lbs, he needs only 2 cans of Fancy Feast a day, or 1 5.5 oz can of Merricks. He also gets various low carb treats and sometimes half a Fancy Feast appetizer throughout the day, so he does get a few extra calories that way.
If a cat is losing a lot of weight
because of the diabetes, you don't want to restrict calories and try and do want to keep weight on her. However, if the cat is losing weight because you're feeding a low carb canned diet instead of high carb dry, that is perfectly natural healthy as long as she's not losing weight too fast. Here is a great guideline to help you keep an eye on it:
http://catinfo.org/?link=felineobesity#Safe_Rate_of_Weight_Loss.
One thing that is sticking out to me is that you say she is 15 lbs but underweight? That seems a bit unusual to me, as the average weight of most female cats is 8-10 lbs. 15 lbs is normal size for a female for a larger breed like a Maine Coon or a Bengal. I'm wondering if maybe what you think is "bony" or underweight is actually her reclaiming a healthier figure if she has been obese in the past? Trust me, sometimes it's hard to judge and get used a cat's natural figure. I know that when Bandit first got down to around 15 lbs (formerly 20+ lbs), I thought I must be underfeeding him. Sure enough, after a trip to the vet where I voiced my concerns about his weight, I discovered not only was he NOT close to being underweight, he needed to loose 2-3 more lbs. :roll: Part of what threw me off was the saggy skin over his belly. Like with obese people, sometimes there is excess skin after a large weight loss (which is why many people end up getting tummy tucks after a large weight loss). Also, you should be able to feel their shoulder blades, and their ribs (with a thin layer of padding). Here's some advice on how to evaluate weight:
http://catinfo.org/?link=felineobesity#Is_Your_Cat_Overweight, and a body condition chart:
http://richmondroadvetclinic.com/sites/site-3991/images/cat20body20condition20chart.jpg
Now, it's very possible she is an unusually large framed cat for her breed and 15 lbs is normal for her, but I would double check that with the vet. One thing that vet schools are very good at teaching is assessing the body condition of animals. :smile: