I like to let a tricky problem breathe for awhile and think about it in the background of more turbulent tasks or on my walks. If all that's left when I begin working on it is organizing the solution into something real, I find flow pretty available.
I started on changeable territory billing at hour 10 in the work day but the flow was so strong I went four hours on it.
I think the main thing for me is not needing to task switch between research/investigation/ideation and organizing the solution.
I agree that batching related tasks is key so as to reduce the cognitive dissonance experienced when switching from one task to another
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