Like any system, you need enough slack. That slack makes you resilient over the long term. It gives you what you value in life. Autonomy, relatedness, and competence to whatever you set your goals towards.
I hate the mentality of "if it's not in a calendar, it's not happening" because this is the opposite of slack. There's a beauty in serendipity when it comes to not being busy. It opens more doors than closes them in my experience.
I would argue that the optimal level is negative in that you're not "busy" by the definition, but you are tackling your top goal until it is accomplished and then to the next one. You won't even make a dent in it if you're constantly distracted by small immediate things. You have to learn to set boundaries and say no. It's the only way things actually get done.