1: Make one clear difference sufficient for simple arms to avoid conflict. If it was good enough for the Earl Marshal's Court of England in 1389, it's good enough for us.
2: Make one clear difference sufficient to avoid conflict with arms registered by individuals in other kingdoms. Except for very famous and important arms, medieval kingdoms did not try to prevent conflicts with arms born by families in other kingdoms, even though a significant number of people actually did move to other kingdoms. Sir Walter de Manny, Sir Otto de Grandson, and Christine de Pizan are three examples.
Excessive requirements to avoid conflict greatly impoverish the available repertoire of simple arms.