PPS:

Here is a diagram that somewhat illustrates the calculations above



Note that the extension also marks the southern edge of the ceiling of the KC. If the ceiling of the KC is 82 cubits plus 11.18 cubits above ground level, and if the floor of the grand gallery crosses the midline of the pyramid 1.618 cubits below the 82 cubit height of the floor of the KC, and if the southern wall of the KC is 26 cubits south of the midline of the pyramid, then the slope of the line from the end of the floor of the grand gallery to the southern edge of the ceiling of the KC will be 11.18 plus 1.618 = 12.798 rise over 26 run.

12.798/26 = .49223

/tan .49223 = 26.208 = 26 deg 12.47'.

The figures in the original post for the vertical and horizontal measures of the ascending passage/grand gallery are 1459.62/2962.65 = .4926

Petrie "gives 26º 12' 50" for mean angle of both passage and gallery together", very close to the angle above, produced by the 1.618 cubit measure of the end of the gallery floor below the floor of the KC and the ceiling of the KC being 11.18 cubits or sq rt 5 x 5 above the floor.