I personally allocate the work day into one 4-hour block as my "big pipe", one 2-hour block as my "little pipe", and let the other 2 hours be "fluff". For limited intervals, I can combine the two pipes into one "mega pipe", to get a "big pipe" task done faster, but it takes a toll over time. The "big pipe" can also theoretically be split into two more temporary "little pipes", but in general, I usually want to have the "big pipe" around to get something important done, and multitasking on three things creates too much organizational overhead, so that also takes a toll.

"Fluff" time is for sitting in meetings or writing reports, as work functions, or for just looking busy. I intentionally avoid doing real work that requires any focus or creativity in the fluff slot. After six hours of real work, any more time spent doing the same thing starts to become counterproductive. So just leave it for tomorrow.

I have been more productive while explicitly dedicating 2 hours a day to getting nothing useful done, than when trying to stay focused on one thing for 8 hours, and ending up goofing off, getting sidetracked, or procrastinating for some amount of time anyway.

That's the theory, anyway. My current employer doesn't keep my pipelines reliably filled. During the dry spells, I have a hard time expanding the fluff slot to more than two hours, and end up doing stuff like coding stupid CLI games, or vectorizing the company logo, or setting up a new vanilla VM that I might not ever use.