Back in the day.....disk management was an absolute necessity for performance. I get a chill thinking about all of the set-up and management chores we used to have to go through. Nowadays, it is all easy street.
Personally, I never let my boot/system drive get anywhere near to full. The only thing I keep there are my OS, System Directories, Swap, Applications, and most libraries. For my system, this chews up about 115 GB, leaving 112 GB free. That is plenty of room for adding new applications.
I also like to have one drive dedicated for working space, and another drive for archiving completed work. Finally, I have external drives for back-up. This may seem redundant, maybe even obsessive. But the last thing I want to worry about is drive space, or losing work.