I’d suggest reading Don Reinertsen‘s “Principles of Product Development Flow.“ Depending on the amount of variation in the work, you may need as much as 50% slack time to have a smooth development flow. You don’t have to be idle during that slack time, of course. You could spend it learning, reading, taking classes, giving classes, Etc. Even playing cards, though, is a better use of your time than artificial busyness. Firefighters have as much as 99% slack time in their schedule. Last thing you want to hear when your house catches fire is that all the firefighters are busy, so won’t be able to get to you for a few hours.

Source: x.com/allenholub/status/1819432500357484991