I like to think of my calendar as available units of commitment rather than open slots of time.
When planning out my week, I schedule blocks in terms of the commitments I want to make to their intended outcomes. I consider not just return on effort but return on energy invested.
Before considering a potential one-hour meeting, I ask whether it really makes sense to use up 10% of that day’s commitment units on it. I think the same way about blocking out commitment units for projects. What will it accomplish and why does that outcome matter?