Johnson Level & Tool Mfg. RAS-1 - Manual (Page 26)

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distance from the edge of the last common rafter to the
intersection of the hip and ridge, or the valley and top
plate (for a valley jack). This measurement is called (P)
in Figures 14 and 15. Now subtract the measurement (P)
from the spacing you are using (W). The result (W – P) is
the distance from the intersection of the hip and ridge to
the first hip jack or the plate corner to the first valley jack.
Look in the table for this distance and find the length to
deduct from the common rafter length. In our example
again, let us take a measurment of 12" from the common
rafter far side edge to where the hip intersects the ridge
(W – P = 24 – 12 = 12). Therefore:
FIG. 15 Top View of Valley Jack Rafter Spacing.