The problem is that the nutritional value for 8 oz of raw chicken breast is the value on the package. Once you cook that 8 oz into 6 cooked oz, you now have a food which your body is now able to extract more calories per oz than what is stated on the package.

How many more calories? Enough to make up for the reduction in weight? It’s not currently practical for the average person to determine this on a meal-to-meal basis.

I agree an estimate is certainly better than nothing and perhaps more so it’s important to be consistent in your approach so you can have less variables to account for.

Here’s another excellent article on the subject: https://www.sciencemag.org/news/2013/02/have-we-been-miscounting-calories