Bodyweight loss vs calorie shortfall
You hit upon several ideas in your post.  First, you are absolutely correct that body fat is composed both of lipids and water, so a 10kg weight loss does not correspond to 90,000 kcals.  The most commonly accepted figure is 7700 kcals per kg of body fat lost (Hall, 2008), but this is a rule of thumb at best.

The translation from caloric deficit to body fat loss is fairly complex, and is the result of feedbacks between metabolism, weight, and other responses.  The National Institutes of Health researchers built a fairly intricate model estimating the actual responses as a differential equation (simulation and references available here), which would probably be a good place to start.

If you're inquiring not from general curiosity but with an interest in developing your own nutritional plan, keep in mind that nutrition is a lot more than merely calories.  It is critical to get macro and micro nutrients, of course, but other factors such as dietary fiber play an important role in the health outcomes.