Place the spirit level tool between the two fixed points and mark the horizontal lines of the two points, which are called a and B lines. Move the spirit level tool to one end of the fixed point and mark the horizontal lines of the two points, which are called a 'and B'. Calculate if A-B ≠ a '- B', align the horizontal wire of the telescope with half of the deviation. Use the calibration needle to adjust the upper and lower screws of the level gauge until the horizontal bubble of the tube coincides. Repeat the above until equal.