Custom clef and incipit spacing for Gregorian chant

In LilyPond 2.24.4, you may want to reduce the horizontal space after a Gregorian clef. You can do this with \override Staff.Clef.space-alist.first-note = #'(minimum-fixed-space . [insert your preferred spacing]). Note: LilyPond 2.25 features improved spacing for extra-slim clefs.

And if your incipit has a four-line staff, but your music uses a modern five-line staff, you may want to add some white space between the incipit and the modern staff. You can do this with \stopStaff and \hideNotes.

\version "2.24.4"

incipitMusic = \relative c {
  \override Staff.StaffSymbol.line-count = #4 
  \clef "vaticana-do3"
  \override NoteHead.style = #'vaticana.punctum
  c1 
  
  % The following line helps with spacing between the first two neumes: 
  \hideNotes c \unHideNotes 
  
  e 
  
  % The following line helps with spacing between the second neume and the end of the visible four-line staff: 
  \hideNotes e \unHideNotes 
  
  % The following line effectively prints white space between the incipit and the modern five-line staff: 
  \stopStaff \hideNotes g a 
}

ictus = #(make-articulation 'ictus)

chant = \relative c' {
  \cadenzaOn 
  c4 e g\ictus a g2 
}

verba = \lyricmode {
  Sál -- ve, Re -- gí --  na, 
}

\score {
  \new Staff \with {
    \incipit 
      \incipitMusic instrumentName = "" 
  }
  <<
    \new Voice = "melody" \chant
    \new Lyrics = "text" \lyricsto melody \verba
  >>
  \layout {
    \context {
      \MensuralStaff
      \remove Time_signature_engraver
      
      % The following line reduces the horizontal space between the "vaticana-do3" clef and the first neume (c1): 
      \override Clef.space-alist.first-note = #'(minimum-fixed-space . 1.0) 
    }
    \context {
      \Staff
      \remove Time_signature_engraver
    }
    \context {
      \Voice
      \remove Stem_engraver
    }
  }
}