Fat is not needed for magical reasons on a ketogenic diet; it is there to replace the energy no longer being obtained from the glucose in dietary carbohydrate. The body’s preferred sources of energy are glucose or fat.
Unless something is seriously wrong (such as a famine, or someone is starving us to death), the body prefers not to metabolise the amino acids in the proteins we eat. And in any case proteolysis (protein metabolism) is very inefficient, because of the energy cost of removing the ammonyl group and disposing of the nitrogen. Much less yield of ATP from one gram of amino acids, as opposed to one gram of glucose, or one gram of fatty acids.
Since fat has over twice as many calories (on average) as carbohydrate, it actually takes less fat to provide the same amount of energy. (In fact, if I’m remembering correctly, fatty acids yield slightly more ATP per gram than glucose.) Also, fat has the benefit of triggering the secretion of only just as much insulin as is required for survival, whereas the glucose in carbohydrate stimulates a much, much greater insulin response (because too much glucose in the blood stream causes damage). Since we want insulin to remain low, so that we can metabolise some of our excess stored fat, this is a good thing. (The insulin response to protein, by the way, is much greater in the context of a high-carb diet than it is when carbohydrate intake remains low.)
So the short answer is to eat enough fat to satisfy your hunger. You don’t need more than that.
By the way, your appetite may still remain high for the first few weeks on a ketogenic diet. You will find it helpful to eat to satiety during this period, even if it seems like a lot of food. At some point during the third or fourth week, if you have excess fat to shed, your appetite will drop noticeably. If it doesn’t, then we can discuss caloric targets, but in general it is better to let the body determine how much food it needs.