Hi,
that calculation predicts the axial compression/extension of the screw but does not accommodate the fixed mount.

Elements that are logically in series, for instance the ballscrew, the fixed mount, the axis bed: the reciprocal of the stiffness of the combination is equal to the sum
of the reciprocals of stiffness for each part in the series.

1/Scomb=1/Sballscrew + 1/Smount +1/Saxis

Lets say that each of the individual stiffness's is 600N/um then the series combination of the three results in a stiffness of 200N/um.

Thus any individual component may seem adequate, every component degrades the stiffness of the combination or assembly.

Craig