[Albert Moulton]

I keep the same records. For each session I calculate the average dollars per hour per session. I then use the standard deviation function in EXCEL to get the standard deviation of all sessions' average dollars per hour per session.

Now, this isn't perfect, since a 1-hour session will have the same weight as a 16-hour session, but as long as your sessions are more or less similar in length this should give you a decent measure of variance in your average win/loss rate per hour per session. And it's easy to do with excel given the information that you are keeping.