I can't make a recommandation on which space figure to use.
Ideally, it must just be *less wide* than a digit and *not justified*, it must
be *unbreakable*. The ideal space to use depends on the available fonts,
and in practive most texts are coded with NBSP (sometimes a standard
SPACE, but using simply nothing is better than using a SPACE), and the
final space is substituted during formatting before publishing.
If this space *must* be *unbreakable*, as obviously it must, the only spaces that are available for use in Unicode are U+000A NO-BREAK SPACE or U+202F NARROW NO-BREAK SPACE.
All other spaces (of size other than zero-width) in Unicode indicate allowable line breaks.
From Unicode Standard Annex #14:Line Breaking Properties at http://www.unicode.org/reports/tr14/tr14-13.html:
2000 EN QUAD 2001 EM QUAD 2002 EN SPACE 2003 EM SPACE 2004 THREE-PER-EM SPACE 2005 FOUR-PER-EM SPACE 2006 SIX-PER-EM SPACE 2008 PUNCTUATION SPACE 2009 THIN SPACE 200A HAIR SPACE 205F MEDIUM MATHEMATICAL SPACE
The preceding list of space characters all have a specific width, but behave otherwise as breaking spaces. In setting a justified line,
normally none of these spaces, except for THIN SPACE when used in
mathematical notation, will change in width. See also the SP property.
Accordingly in French U+202F NARROW NO-BREAK SPACE is the character to be used before or after certain punctuation for narrow spacing where no break is allowed, not U+2009 THIN SPACE or any other spacing character.
Jim Allan
