I don’t care about the length of the mark, but I did find this idea useful. Prone to excessive detail, I often find myself with a parenthetical inside of parenthetical. The developer in me insists on 2 closing parentheses. But it looks weird and nerdy. Although, using an em dash instead is probably just as nerdy.

> Dashes are used inside parentheses, and vice versa, to indicate parenthetical material within parenthetical material. ...

> The bakery’s reputation for scrumptious goods (ambrosial, even—each item was surely fit for gods) spread far and wide.