Re: FMS VNAV and altimeter setting.
And don't forget that you don't just let the plane get on with the descent right down into the spuds. You ""Arm" the FMS through the Altitude Select function (located, in my aircraft on Flight Mode Panel) and you enable further descents as and when you near the vertical profile steps of the approach, assuming you have been so cleared. If ATC don't specifically clear you to an altitude, the approach plate may give some guidance as to when and where you should select QNH.