Why does there need to be a metric? People with management experience can decide these things based on intuition. It's practically impossible to quantify anything in this industry, but 'data-driven decisionmaking' is fetishized to the extreme. Ultimately it just becomes another buzzword and method for people with little or no experience to override people with experience.

Phrasing the whole discussion in terms of 'metrics' to justify the common-sense conclusion that junior people need more direct supervision is clearly sugarcoating it for junior recipients for whom the message is intended.