Stadelmann was apparently basing his 45* measurement on the slope found on some of the remaining casing stones. Josef Dorner (in "Neue Messungen an der Roten Pyramide" in: Stationen: Beiträge zur Kulturgeschichte Ägyptens. R. Stadelmann gewidmet) did much the same and arrived at an average of 44°44' for the east face (which he then rounded off to 45* in his computations).
Petrie (in A Season in Egypt, 1887 - p.27) gives his measurement of the slope of the core masonry for the north and west sides to be 44*42' and 44*41' respectively, and for the east and south sides to be 44*32' and 44*30' respectively.
Perring (ca 1837) is, I believe, the one who first arrived at the 20/21 (43*36') slope assessment - and this was later quoted by Petrie in his Pyramids and Temples of Gizeh (1883) - although, as just stated, Petrie found this to be about 1 degree too shallow when he made his own measurements in 1887.
I do not doubt that this pyramid was designed to incorporate a multplicity of correlations. What I am saying is that there is this multiplicity, and that it is very cleverly nuanced in order to simultaneously accomodate (or reflect) the various desired factors.
Furthermore, I am saying that all of these theories are very nice and interesting, but until one can prove any of them they remain only theories. We must see in what ways we can devise a means to test them, and so quantitatively move the game forward.