The measure as written
In THE EQUILIBRIUM OF DESIRE AND NECESSITY, under "Hunger", the equilibrium between desire and necessity rests on the endurance level, which each person comes to know:
(An equilibrium can be achieved by suppressing the desire in accordance with the endurance level.) (A person comes to know his own endurance level step by step, i.e. by holding back a desire a little at a time and noticing how far he can go without falling into psychological stress, along with the advice of his sincere relations.)
The same paragraph gives its measure, in the second half of the sentence whose first half fixes the necessity basket:
… while the endurance level shows in the performance curve, i.e. the point where an individual’s output starts falling as the load rises.
This sub-section sets out how the endurance level is read: as the point on the performance curve where a person's output starts falling as the load rises, known to the person step by step, with the advice of the sincere relations.
What it measures, in plain words
The equilibrium between desire and necessity is kept "by suppressing the desire in accordance with the endurance level". The endurance level is one of a person's HSPs (see Recognizing a person's HSPs, at the start of this part): the amount of societal stress that a person can bear without being psychologically depressed. In RESPECT IS NOT COSTLY, THE INGREDIENTS ARE UNIVERSAL, it is "the endurance level that decides how far one can suppress one’s own needs", and "endurance sets its limit".
How a person comes to know it. Step by step, "by holding back a desire a little at a time and noticing how far he can go without falling into psychological stress", "along with the advice of his sincere relations".
Where it shows. On the performance curve: "the point where an individual’s output starts falling as the load rises". Up to that point, more load does not lower the output; beyond it, the output starts falling.
Within the limit. The desire is suppressed in accordance with the endurance level, and not beyond it. The same limit is kept when the society persuades a person to change position, "keeping in view the Endurance level of the child" in the example (see Recognizing a person's HSPs).
The method.
· Knowing it. Each person, step by step, holding back a desire a little at a time, with the advice of the sincere relations.
· Reading it. The point on the performance curve where the output starts falling as the load rises.
· Keeping within it. The desire suppressed in accordance with the endurance level, which sets its limit.