以常衡变

以常衡变 is usually explained as getting a feel for what is 'normal' in the population at large in order to know when a certain individual is 'abnormal'.

I've never seen it interpreted as knowing what is normal for a certain individual, even though this should be an important aspect of diagnosis.

Perhaps it is because it is difficult to get to know individual patients in modern TCM+WM hospitals. Or perhaps because knowing what is the normal range of a certain index across a population (e.g. blood pressure) is an important aspect of practicing WM?