So, asking my question first, in the macro nutrient pie chart available both on the website (at the end of the Food page) and in the app, do I take it that the proportions are by weight as figures elsewhere seem to be by weight (in grams in my case)? How does one convert from macro nutrient proportions by weight to the proportion of calories obtained from each of the macro nutrients and vice versa?
The background to this is that there is an article in the New Scientist looking at appetite: https://www.newscientist.com/article/mg24632831-400-you-have-five-appetites-not-one-and-they-are-the-key-to-your-health/
You'd need a subscription to read it but, in summary, research has been conducted among various species from locusts to primates and humans and found that, when allowed to browse a variety of food, individuals eat the right amount of all the macro nutrients but when choice is restricted to a single food they eat the right amount of protein regardless of whether that means eating too much or too little of fat and carbs.
The implication here is that we are more likely to be hungry and struggle not to eat more calories than we use if we eat too little protein. This also links with the experience of a colleague at work who reported that loosing excess weight was easy - as long as he kept his protein intake up he didn't feel hungry.
So, obviously, I would like to check my protein intake, compare with the figures in the article, and see how that affects hunger or saity but that article is working in proportion of calories from protein and, as I said, I suspect My Fitness Pal is working in proportion by weight.