shortInstrumentName vertical spacing

[David Sumbler]

In string parts, I often use more than one stave where a section is
divisi.  I write, for instance,

\set Staff.shortInstrumentName = "2."

when one player (or sub-section) is required, or

\set Staff.shortInstrumentName = \markup \column { "3." "4." }

if two players need to play this line.  The default settings are fine
for this.

But I have a stave now which applies to 3 players.  If I put 

\set Staff.shortInstrumentName = \markup \column { "2." "3." "4." }

it takes up rather too much space vertically, so I want to move the
numbers closer together.  But nothing I have tried so far makes the
slightest difference.  My latest version is:

\set Staff.shortInstrumentName = 
  \markup { \override #'(baseline-skip . 6)
            \column { "2." "3." "4." } } 

I felt sure that this would work, but it doesn't.  ("6" is a value just
for testing - obviously the value I want actually to use will be much
smaller.)

What am I doing wrong?

David