I am counting from my highest ever weight on my tracker on this site (352), but my surgery team and primary care doctor count from my pre-op start weight (321). There's a 2 year and 30lb gap for me, but I'm counting that for me since it was work to break old habits during that period, which helped me set the pace to be more successful post surgery.
As far as sticking to a plan, as others have mentioned, they do use average numbers. Everyone will fluctuate a little above or below at each stage. Also the weight loss progression will look more like stairs/steps rather than a straight line. You can expect a period of weight loss, followed by multiple plateaus. That's just your body freaking out over an unusual weight loss event, and recalibrating things to make sure you're not starving to death. Since it can't know you're doing it intentionally, it will retain fat and fluids as it panics, then release them once it realizes you are safe. Just stay the course, and don't get caught up on seeing a steady loss, otherwise you'll end up worrying over nothing (like I tend to do haha).