You would use it to determine which part of your machine needs service

You will need a precision level and a calculator

Step one

Level the machines bed ways for datum . We will assume the machines bed ways are level with the horizon in the following procedures.

Step Two

Take the readings and plug them into the formula

Be especially careful of the signs +/-

Formula

(Position A Reading) - (Position B Reading) /2 = Saddle Error 

(Position A reading) - (Saddle Error) = Rotary Table Error