Bruce, the link that TheOrangePimpernel posted is one way to see it - the calculation was roughly 28 to 34 calories per day per pound of body fat. The study was based on Ancel Keyes’ Minnesota Starvation experiment, and I wish there were better, newer experiments to confirm, broaden, nail down the results, etc. Keyes didn’t look at fasting people - he restricted a bunch of guys to 1570 calories for six months. It would be good to see what happens with people when truly fasting.
It does make sense that there is a limit on how much fat the body will/can burn in a day. The need to conserve energy from a worst-case situation, i.e. needing to keep the body alive, would explain why we are evolutionarily wired this way.
I think you’re right about relatively lean people not having enough fat to supply all their energy needs, and in general they do seem to have a relatively hard time fasting for multiple days.
Our fat actually is very close to 9 calories per gram. And indeed - our adipose tissue is 87% fat. 454 grams (one pound) x 9 calories x 87% = 3555. So, very close, to begin with.
Longer-chain triglycerides are more calorically dense than shorter-chain ones, and we have a mixture of them in our stored fat. But it’s not too mysterious - over 90% of our stored fat are the triglycerides with 16 or 18 carbon atoms, palmitic and stearic acid.
When fasting, most people lose right about a half pound of fat per day. This is quite consistent, with bigger people and people with a LOT of fat losing a little more. Some extremely obese people, on long fasts, have lost 0.6 or 0.7 lbs per day of fat.