It sounds like you want more information on calculating your bmr, tdee, and tracking your intake so that you can estimate your projected weight loss? It also sounds like you're interested in losing at a healthy rate (like 1-2% a week)?

Honestly you may want to check out the FAQ on the side bar of a sub like /r/loseit. They break down calories in/calories out and will explain how to calculate what you need and track your intake. I'm on mobile and can't see your flair, but it's very likely your tdee is at least in the 1400s, which, as another user mentioned, means you could not exercise at all, eat 900 a day, and lose a pound a week.

If you don't trust tdee/bmr calculators, I've seen spreadsheets posted here and on xxfitness (honestly you could find them by googling) that allow you to input your historical caloric intake and weight to determine what your tdee actually has been. Note that your tdee will decrease as you lose weight and will fluctuate based on your activity levels, but you can compare historical data about yourself to tdee calculators to get a sense of which method of calculation is most accurate.

Personally, I have a desk job but walk about an hour a day as part of my commute so just list myself as sedentary for tdee calculators. I compared like four months of weight and calorie data from a spreadsheet (calculating my actual tdee) to a tdee estimate from a calculator and found the calculators to be accurate within around 100 calories. Note that my bmi is 15, so my weight is probably what you would consider to be an "outlier." Also note that I don't track or way back exercise calories.

As far as tracking calories that you purge, I don't b/p so I don't really know much about that. My understanding though is that you begin to absorb calories as soon as you eat something and the longer you wait to purge, the more calories you will have absorbed, plus it's very difficult/impossible to accurately gauge what percentage of calories you've actually absorbed (I could be wrong though--just repeating what I've read). I've seen people here say they track 100% of their binged and purged calories just in case.

As far as actual tracking, I use the Lose It app. It will calculate your tdee and deficit for you. I prefer it to my fitness pal because it doesn't yell at you and suspend your account for eating under 1200.

Sorry if this is way longer or more technical than what you were looking for--good luck though!