The built-in M stat is based on information from the end of the last hand, so "session only" wouldn't have any effect.
Bear in mind that the M stat will not be correct for one hand after the blinds increase, and to be accurate you need to configure your Refresh Rates to update every hand - see the
Tutorial: Configuring HUD Options (e.g. configure a single range 1 - 999999 hands, to update every 1).