Yeah, you're missing conversion to hours and/or minutes. Or, probably you can say that *I* missed it. :-)
Problem here is that the TD currently doesn't allow you to change the type of the value that the Overall Score(s) produce. So it's going to be displayed as a number. The Playing Time column displays as hours, minutes, and seconds (6h 14m 33s), but there's no way to tell the Stats that the Overall Score column should be treated as a time value. So it's just going to display it has a number.
(playingTime / buyins) / 3600
Now it should display in "hours". So, for example, one of my players had an Avg Playing Time of "14,106.5000". Using the updated formula it becomes "3.9185", or 3.9 hours.
Not perfect, but better ... ?