While Tiago Forte argues that a life area’s standard is difficult to articulate and assess, I find that an area’s standard can be articulated, with benefits.
It can be articulated as a description of a seemingly recurring end state, which is different from a goal, a description of a non-recurring end state. I say “seemingly recurring” because, to be philosophically precise, all end states are non-recurring, much as events cannot be repeated per se. Mental states are similar, not the same (see pursuing a mental state as a purpose requires reconstruction).
To say it in another way, we can write standards much like how James Clear writes his goals: He defines what he is doing consistently, not what he has achieved.