AlliumCepa: a graph is just a series of results, so my formula could certainly be graphed if you want to show history.

It isn't meant to be clairvoyant, so while doubling the number of games *may* not mean twice as slow, it's the best guess with the available data. Of course you could discount time where no moves are available if you want to be more accurate -
moves made / time with at least one move available, over, say, the last month.

"moves per day" = moves / days. It *is* an average. Otherwise I would say "moves made today" or something like that.