An app that takes in a recipe and scales down the quantity of ingredients to be able to make just enough of it for one person.

1/3rd an egg

Should have BMI entry then.

Most recipe keeping app already do this. Recipes often have the number of servings mentioned, so recipe apps let you scale the serving up or down and update the amounts since It's a simple scaling operation.

Yes, you will get results like 1/3rd of an egg, but that's up to you as a cook to resolve, either really use just 1/3rd or decide that maybe use 1 whole but small one

you mean this thing called "being able to do 4th-grade math"?

sorry, but i doubt there's a target group big enough that satifies both "being dumb enough to need this" and "being smart enough to use an app"