Flow, boiled down to 8 rules

In a footnote to this post by Ann Althouse (my mom). When someone asked AskMetafilter for examples of psychological "rules of thumb," I gave a shot at boiling down Mihaly Csikszentmihalyi's Flow into one rule:

You will be happiest if you do an activity that poses appropriately increasing challenges allowing you to continually engage with the activity and apply your increasing level of skill to effectively tackle the challenges.

(That was my off-the-cuff version; I left out some of the elements.)