Thank you for the replies.
So in the cruise we disengage the A/T and set turb n1 from the fmc. okay
In the climb/ descent if we encounter severe turbulence we set the speed to 280kts/.76 and also disconnect the autothrottle.
What i am having difficulty with is why in the descent/climb the turb n1 figure(which in the cruise provides near optimum penetration airspeed) cannot be selected.