Below is a provisional list of conceivable rotations, corresponding to rotations.json. Not all of these rotations have been attested yet, and we have erred on the side on anticipating more potential rotations than may occur in actual texts. The motivation is that we do not wish to place undue limitations on encoding projects in the near future. Even so, it is very likely that encoders will come across rotations that have been overlooked so far.
There is often a choice between rotation counter-clockwise or rotation clockwise in combination with horizontal mirroring. Some attempts to be consistent are appropriate. For signs that have an iconographic "above", that "above" can vary most frequently between top and left. For example, a flat sign like a platform or papyrus roll is rotated most often counter-clockwise, so that the top side becomes the left side. A flat sign like M3 (wood) is most often rotated clockwise, so that the tip of the branch changes from being on the left side to being on the top side.
An exception seems to be if a sign points down, like the standing arrowhead T22; a quarter-turn rotation clockwise then makes that sign point to the left. Similarly, bows T9 - T10, which in that orientiation would shoot an imaginary arrow in the downward direction, would be rotated most often clockwise, to shoot an arrow in the leftward direction.
Otherwise, where there is ambiguity between rotation clockwise and counter-clockwise, we have opted for clockwise in the case of signs that in their most common form are wide and low, and for counter-clockwise in the case of signs that in their most common form are tall and narrow. If a potentially rotated shape can alternatively be obtained using mirroring, then mirroring is preferred.
The diagram has arrows that depict multiples of 45 degrees. Such rotations correspond to one of 7 variation selectors. Some arrows are followed by a positive or negative number, which denotes an additional angle in the clockwise direction (if positive) or in the counter-clockwise direction (if negative). For example, an arrow depicting a 45 degree rotation followed by -10 means that the way the rotated sign should actually be depicted is with a rotation of 35 degrees. To enable automatic conversion to and from other types of encoding that represent rotations with finer granularity, this offset should not be less than -22 or greater than 22.
Some of the rotations in the below list probably need to be removed. In particular, signs that are already diagonal but are rotated by say 30 degrees to be diagonal at a slightly different angle do not merit to be distinguished. These cases are temporarily here for demonstrative purposes.