Z axis travel and steps

Being new to Easel end CNC routers the following is a bit out my field.
During machine setup the OEM shows to use M8 threaded rod.
Sometimes the Z travel is correct. 10 mm is 10mm but mostly it goes down to 2.5 m travel.
This is purely steps in firmware but how do I change it?
Motion controller is Marlin & gShield, X-carve type machine. Nudging me into the correct direction would help a lot.


If you want to change the steps per mm this is how you do it.

Calibrating my X-carve 1000 - X-Carve - Inventables Community Forum

If you need to change direction:

How do I INVERT the Y and Z Axis of my GRBL CNC? Arduino UNO - CNC Routers / DIY CNC - Maker Forums

GRBL_Settings_Pocket_Guide_Rev_B.pdf (diymachining.com)

Marlin? Are you sure?
What machine do you have?
What is the Z-axis like?