John, your numbers look OK. I wasn't certain yesterday exactly what "mortise layout" you were after and didn't have time to explain the math anyway.

The easiest way to think of your saw bevel along the miter line created by the Jack Rafter Side Cut angle (Hawkindale Angle P2) is to think of the intersecting planes. The surface of your jack rafter and the plane formed by backing the valley are equivalent planes. The plane formed by cutting the compound angle on the jack rafter matches the the side (plumb) face of the valley. The blade bevel to make the cut from the face of the jack rafter set in the plane of the roof must be the same as the bevel to back the valley rafter: Hawkindale Angle C5.

There are different methods of analysing the relationships in a compound angle. Here's a link to a Trigonometric Solution of a Compound Angle . The angles may also be determined by Development . This development assumes the angles on the faces of the stick are known. If a miter angle and the blade bevel (or the dihedral angle) are known the angles may be solved as per this Development of Miter Angle on Upper Shoulder of a Hip-Valley Rafter . (Note the title! The stick being developed is NOT a jack rafter - however, the process is the same. The givens for a jack rafter would be the plumb cut angle and the 45° Deck Angle).

The formula for the rafter mortise spacing along the Valley is:
Spacing O.C. along Ridge Line/sin Jack Rafter Side Cut Angle
or Spacing O.C. along Ridge Line/sin P2
Substituting the numbers in the formulas:
48"/sin 35.26439° = 83.13844", or about 83-1/8".
The easy way is to use this Excel Unadjusted Hip-Valley Dimensions "cheat sheet" ... here's a link to a web based Hip-Valley Framing Calculator .

Hip-Valley Roof Framing and Joinery Math