Does the vertical scale of the spherometer have any use? I only depend on the circular scale of the spherometer.
The vertical rod on which the vertical scale is engraved acts as a fiducial mark (point of reference) when using the circular scale.
The vertical scale tells you how many complete revolutions the centre leg has undergone.
If the pitch of the thread on the centre leg is 1 mm then one complete revolution of centre leg corresponds to a vertical distance of 1 mm which can be read off the vertical scale, the subdivisions of 1 mm being read off the circular scale.
The zero of the vertical scale with the circular scale also reading zero should correspond to the spherometer being placed on a flat surface.