One's diet, in my opinion, shouldn't be an exact science, but something that becomes intuitive and flexible. I don't mean flexible in the sense that you're like, "Oh...I can have a little more of this and a little more of that because it's not that much, right?" I mean flexible in the sense that we cannot control every single thing in our lives. In a situation where someone else is preparing your food and you can't precisely measure it, make sure that the things they do put into it are lower calorie, higher nutrient foods. And if you can tell that they put a little too much of whatever in it, just leave a little on your plate at the end.
I know that there is the idea of the clean plate club, but as someone once said...all food goes to waste eventually, it's just that some of it ends up in the landfill, and some of it ends up in the toilet, but if you eat more than you need, not only does it go to waste, it goes to waist. Or, if you end up eating just a little more (because you can't NOT have a clean plate), then just exercise a little bit more. We should feel empowered to adjust our behaviors and choices based on real life. There is no one perfect or right answer. Also, the more you practice measuring your own food portions at home, the easier it is to estimate by eye in the real world outside the kitchen.
Even with an apple, not all apples are created exactly equal, but when we record our calories/nutrition, we don't put the kind of apple, the exact amount of the apple, etc...we just put "apple" (maybe "medium apple). We record what a medium apple usually means in terms of calories, based on reports. Even if you don't know the exact perfect recording of calories/nutrients, it's better to be conscious of it and do a good guestimate. It still makes a difference vs. not recording it or being conscious of it at all.
Basically, just do the best you can and get a sense of how your body handles the food. If you eat there regularly to save money, you'll see soon enough whether you gain or lose weight. If you gain, eat a little less, or choose something other than an omelet. Or do 15 minutes of exercise in the evening. If you listen to your body, you'll figure it out.