A smidgen is precisely defined ... just not in relation to another physical quantity. (And any one smidgen is not necessarily the same as another.) The smidgen

A smidgen is precisely defined ... just not in relation to another physical quantity. (And any one smidgen is not necessarily the same as another.) The smidgen is defined by its effect. A smidgen of X is the precise amount of X that produces the desired taste. Obvious really.