Pentax PLP-601 Instruction Manual page 36

Read the target plates on the walls (A,B) to detect the reference points.
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If the readings of the target plates differ less than ± 5 mm, the
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instrument needs not to be adjusted.
If more than ± 5 mm, proceed to the following (Adjustment)
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procedure.
Wall A
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Appr. 20m
Target
Detector
X side in 1
Appr. 20m
X side in 5
Wall B