If you have a way of measuring your body fat to a reasonable degree of accuracy, you can ration your food by knowing how much stored body fat your body can convert into energy per day. Your body can convert roughly 0.9% of your body fat into energy in a day. So if you're 180lbs with 33% body fat, you have 60 lbs of fat on you. You should be able to convert ~0.54lbs into energy, which at ~3500 kcal per lbs comes out to 1890 kcal being the maximum calorie deficit you can overcome with just stored body fat. Subtract that from your daily calorie expenditure (another messy estimate) to get how many calories you need to consume to avoid muscle breakdown and energy crashes.
Keep in mind that every person's body and metabolic system is unique so you'll want to put in a generous error buffer. I'd cut the calorie deficit by half, at least to start with. Also notice that each day your total stored body fat will be less, so the number of calories your body can extract from that stored fat will also be less.