Johnson Level & Tool Mfg. 40-6902 - Manual (Page 9)

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Suppose one of your points is below the line of sight and the other
above (Fig B.), C is 4 feet 6-1/2 inches below the line of sight, and
point D, the underside of a floor beam is 7 feet 9-3/8 inches above
the line of sight (the latter reading having been obtained by holding
the rod upside down with the foot of the rod against the beam). D is
then higher than C by an amount equal to 4 feet 6-1/2 inches plus 7
feet 9-3/8 inches, or a total of 12 feet 3-7/8 inches.
5.5 Measure the difference in elevation requiring more than
one set-up
If two points are either too far apart or at too great a difference of
elevation to be observed from one set-up, the procedure shown below
is recommended. This example assumes that you want to find the
difference in elevation between points A and D. To make the finding of
this difference simple, use the convenient terms plus (+) sight and
minus (-) sight and carry the readings at each set-up as shown.
The difference of elevation between D and A is found by taking the
difference between the sum of the plus sights and the sum of the
minus sights. If the sum of the plus sights is larger, the final point is
higher than the starting point. If the sum of the minus sights is larger,
the final point is lower than than the starting point.
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