Eating back half of exercise calories?

MFP is configured to give you a calorie deficit without any logged exercise, based on routine daily activity.

You then log exercise calories.

Then it expects you to eat that number of calories more food ("eat back") to get back to the original deficit.

If the logged calories are an overestimate this can be detrimental, so some people "eat back" half of them to be sure they don't reduce the deficit.

I pretty much eat all mine back, but I don't log exercise except strength training, and use my fit bit adjustment instead. To me the numbers MFP gives for exercise are usually off target, since I don't burn a ton of cals bc I'm more compact.

MFP gives you a calorie goal that includes your weight loss deficit...you're not trying to create your deficit through exercise with MFP. The activity level in MFP is just your day to day stuff WITHOUT exercise...as such, exercise is unaccounted for activity...to account for it, you log it and get those calories to "eat back" People often do 1/2 because it is so difficult to estimate calorie burned and databases and other calorie burn calculators can substantially overestimate burn.

I will use my own numbers for a little math lesson. I maintain WITHOUT exercise on roughly 2300 - 2400 calories per day. To lose 1 Lb per week I need to be around 500 calories less than that per day...so lets say 1850. Now lets say I go run 3 miles every day and burn 300 calories...I log that and get those calories to eat back...so now my goal is 1850 + 300 = 2150...but I still maintain that 500 calorie deficit because my maintenance calories will have also increased by the same amount.

Just math...other calculators assume some estimate of those calorie upfront in your activity level...MFP does not.