You're overestimating the difficulty here. You don't really need to wait a month either. There's plenty of variance on a day by day basis, but you can still generally see whether the number is trending up or down. And most people tend to cycle through the same foods, so portion control isn't particularly hard.

For instance this is something every single person who's into body building does, because you want to be in a slight caloric surplus when bulking, and then you want to get back into a slight calorific deficit when cutting.