How to decide the quantity of raw material in preparing food for two people and avoid wastage?

Answer 1

Trial and error.
Judge how much you would eat and double it.
Left overs can be used in a fry-up once a week, or frozen for another day.

Answer 2

Most cookbook and magazine recipes are 4-6 as are the online recipes. Nothing wrong with leftovers. And, cooking for 4 usually only costs around 25% more than cooking for 2 so your better off making a little extra and freezing the leftovers.

If you really hate leftovers, just half the recipe.

Answer 3

You sort of have to know how much it will cook down, then work backwards to be sure you have enough (but not too much) for 2 portiosn.