Johnson Level & Tool Mfg. RAS-1 - Manual (Page 23)

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STEP 3. BOTTOM OR TAIL PLUMB CUT: Measure down the
top of the valley rafter from the SEAT PLUMB MARK and
mark the tail length (in our example: 2' 8-7/8"). Be sure to
add the miter allowance in the measurement, if used (see
Figure 13). Make the bottom plumb mark with the square
in the normal manner.
STEP 4. RAFTER PATTERN CUTS: To make the top and
bottom cuts, tilt the saw at 45° (see Figure 13). Also make
the seat miter cuts. Now change the saw tilt to 90° for the
horizontal seat cut. You may want to check for proper fit
and use this rafter as a pattern for the other valley rafters.
FIG. 12 View of Valley Rafter Layout.