Step1：Zero point at your chosen specific distance, such as set the position as followed picture 

Step2：Use the spanner to unscrew the bolt

Step3：Use the spanner to loosen those four bolts 

Step4：After loosen four bolt, then tight black cover to the base

Step5：Tighten the four screws 

Step6：Cover the turret and set this position to the zero point and tight the outside screw