I used to treat capacity as something to prove. If there were ten things to do, I wanted to do all ten. If there were twelve, I would try to fit those in as well. I assumed more effort would mean more progress.
I began to notice that I could be busy all day without giving anything important enough attention. My mind felt scattered, and the people around me received whatever attention I had left. That was the problem I needed to address.
What I mean by N minus 1
N minus 1 is a reminder to question the last commitment I think I can fit in. If I believe I can do ten things well, perhaps I should choose nine. If I can fit five major calls into a day, perhaps four would let me give them the attention they need.
This is a reminder I use, not a formula. I can do less and still neglect what matters. Before saying yes again, I need to ask what the extra responsibility will take from the work I have already agreed to do.
I have seen a similar problem in discussions. When an agenda tries to cover everything, the participants can finish the list without giving much of it enough thought. Removing an item may give an important question more time. The decision depends on what the meeting is meant to accomplish.
Consider the cost before saying yes
An added commitment may mean listening less carefully, rushing a decision or giving less attention to a person who is already relying on me. Those costs do not always appear in the calendar.
I still overfill my days and convince myself that one more thing can be squeezed in. I am learning to pause before agreeing and ask what the extra work will take away.
Look at the next commitment you are considering. Which existing promise would receive less attention if you accepted it? That is the question N minus 1 helps me remember.