Input shaft (worm gear) play

Do you mean by 'flexing in and out' that the input shaft is moving/shifting in and out along its axis, i.e., towards or away from the steering wheel? There is some small in and out movement of the worm shaft (input shaft) as part of the normal operation... it is this movement that activates the valve that directs the pressurized fluid flow for power assist. How much is normal... I don't know the answer. I am pretty sure there will be more movement if the engine and pump are not running.