I don't associate this with flow at all. I'm certainly not in a flow-state when gleefully considering evacuation plans. I'm just enjoying nerding out about it.
Hmm. If I'm calling it the wrong thing, then, maybe I should give an example of me enjoying nerding out to see what I should be calling it.
If I were to step back and think "It doesn't actually matter what the specific stats are for human versions of My Little Pony Characters in a D&D 3.5 setting, no one is going to be judging this for accuracy." then I'm not actually having fun while making their character sheets, and I wouldn't have bothered.
But If I'm just making the character sheets, then it is fun, and I'm just enjoying on nerding out on ...
If it's worth saying, but not worth its own post, even in Discussion, it goes here.