If you have a known bowl and fill it to a known position every day with the same type of food, then you can probably do better than the average for that specific meal. In our research we've found a majority of calories for most people come from when they're eating out and consuming new dishes where they don't know the ingredients or portion sizes.

In the study we gave people a variety of dishes to make their estimate on, some they were familiar with, some they were not.

The professionals were nutritionists who had trained in portion size estimation and were shown 2D images on a computer screen.

For what it's worth, we've had a lot of people who have claimed to be very accurate at portion size estimation from a long history of using a kitchen scale. We've paid many of them to do a quiz to see if they're above average accuracy and they have almost always ended up around 40% accuracy or worse.