\version "2.13.54"
#(ly:set-option 'debug-skylines)

BoxMarkTextIIa = #(define-music-function (parser location MarkText AddText x-align) (string? string? number?)
#{
    \once \override Score.RehearsalMark #'self-alignment-X = #$x-align
    \mark \markup { \bold { \box $MarkText \normalsize $AddText }}
#})

#(define-markup-command (center-at-markIIb layout props mark text) (string? string?)
"x-align on the center of the rehaersal mark"
    (let* (
        (txt (markup #:normal-text #:bold #:normalsize #:concat (" " text)))

        (txt-stencil  (interpret-markup layout props txt))
        (txt-x-ext    (ly:stencil-extent txt-stencil X))
        (txt-width    (- (cdr txt-x-ext)(car txt-x-ext)))

        (mrk (markup #:normal-text #:box #:bold mark))

        (mrk-stencil (interpret-markup layout props mrk))
        (mrk-x-ext (ly:stencil-extent mrk-stencil X))
        (mrk-width (- (cdr mrk-x-ext)(car mrk-x-ext)))

        (x-align (- (/ mrk-width (+ txt-width mrk-width)) 1 ))
    )

    (interpret-markup layout props
        (markup #:halign x-align #:concat (mrk txt)))
    ))

BoxMarkTextIIb = #(define-music-function (parser location MarkText AddText) (string? string?)
#{
    % Alternative using \halign within the markup
    % \halign only has effect if X-offset is set to #0
    \once \override Score.RehearsalMark #'X-offset = #0
    \mark \markup { \center-at-markIIb #$MarkText #$AddText }
#})


\score {
    \relative c'' {

        \clef "G"
        \key c \major
        \time 4/4

    % Option IIa, an override before the markup works as expected
    % I could calulate the value for x-align with a markup script but how do I get the resul
    % in the override?
        \BoxMarkTextIIa #"10" #"Theme from 007" #-0.83
        c1 \bar "||"
        e( f) \BoxMarkTextIIa #"11" #"Theme from 007" #-0.83
        g( e) \break

    % Option IIb, /halign inside the markup.
    % Makes it possible to calculate the value for x-align inside a markup script.
    % But, setting X-offset to "0" leads to misalignment on the clef. See mark 12.
        \BoxMarkTextIIb #"12" #"Theme from 007"
        c1 \bar "||"
        e( f) \BoxMarkTextIIb #"13" #"Theme from 007"
        g( e)
  }
}

\layout{
    indent = 0
    ragged-right = ##t

    \context {
        \Staff
        \remove "Time_signature_engraver" }
}
%%% end of example