I've been tracking my weight and macros daily for just a little over a week now (this time around) using MYPLAN here at lowcarber.org but I don't think of what I write down as gospel since you are right, there are so many variables. Yesterday I cooked in the crockpot "pork shoulder, boneless ribs" as per the grocery store label but of course meat has no nutritional labeling, and lots of meat and fat there. So when entering my data in MYPLAN I chose to enter from the MYPLAN food selections:

"Pork, fresh, composite of trimmed retail cuts (leg, loin, shoulder and spareribs), lean and fat, cooked".

That was certainly as close as I could get! So those were the numbers I used but of course the protein/fat could easily be variable my many grams one way or the other. I figure that's just how it is and I don't obsess over it. Close enough for government work.