>people are on average off by 53% and even trained professionals are still off by 40%. Basically if you want to have a higher level of accuracy you need to use a food scale or something that measure the volume of food, people just can't estimate portion sizes visually.

I can typically estimate them accurately without direct measurement, and with feedback that will tend to make errors cancel out over time. My trick is to note package weights, and divide containers into N equal portions. That is: I decide a target portion size first, and then portion it out.

If the task is "measure out an ounce of butter" I realistically won't be 40% off - because I can very accurately divide a rectangular solid in half repeatedly, and the butter comes in a one-pound package. Similarly, I have a pretty good idea how much grilled chicken is on my plate, because I know how much raw chicken I cooked, because I made a whole piece from a pack of N roughly-equal pieces weighing X (values which I noted when I bought it).