Thanks, much appreciated!
as a 13 rotated by 180° helps me.
I used ^ and v for quartertones 33:32 by my own before seeing this notation elsewhere, because it's an obvious character pair, and the German word for quartertone is 'Viertelton', so for me it was an obvious choice.
make perfect sense for me: The first important books about microtonality that I read were by Martin Vogel, who used / and \ for syntonic comma 81:80 alterations. Like Hermann von Helmholtz (who was in turn inspired by others), he also used overlines and underlines, and / and \ are convenient alternatives that retain the line aspect, while pointing in the direction of the alteration if you read them from left to right - which implies that the stem should be to the right, s.th. the arrow direction makes sense.
is intuitive for me, because it reminds me of the 63/64 accidental used by Martin Vogel, which looks like
, and similar to a 7 rotated by 180° (according to him, it was used "since Tartini's time"). However, it is logical to use
for 54/55 instead, s.th.
, so using a slightly different accidental for 63/64 is perfectly understandable, and the rainbow comparison helps me even more.