+* Add some padding (@code{#:hspace 7.1}) into the definition of your
+custom dynamic mark, so that after lilypond center-aligns it, it is
+already correctly aligned. Drawback: The padding really takes up that
+space and does not allow any other markup or dynamics to be shown in
+that position.
+
+* Shift the dynamic script @code{\\once\\override ... .X-offset = ..}.