Hi Gabriel, Rehearsal marks are almost universally always placed at bar lines, then there's no possibly ambiguity about where they are; and if there is an obvious upbeat (which is highly unlikely to be the case if you're engraving polyphony, but oh well) then it's not much trouble for the director to tell "start on the upbeat of K"... Gould suggests they should either be left-aligned or centered over the bar line, which seems good advice. That said, if you still need to put a mid-measure rehearsal mark, the exact behaviour of self-alignment-x is described in the documentation <https://lilypond.org/doc/v2.23/Documentation/internals/rehearsalmark>, I don't know if that will help you get closer to what you want; I generally find X-offset and if need be extra-offset to be more useful for adjusting the position of the occasional element (be it text, dynamics or rehearsal marks) that ends up being in the wrong spot.
Cheers, Aleksa
