sign padding

What is the best way to achieve sign padding in front of an <mn> such
that the leading space is as wide as a <mo>-</mo>? I tried using
Courier New (monospaced) font along with <mspace width='0.22222222em'>
(the default width of mediummathspace), but that wasn't wide enough. :-
[

Should I be looking at mphantom?

P.S. I'm generating the MathML fragment from a linear string that
contains the spaces, sign (if negative), and number. because my rules
are part of a more general conversion system, I should reasonably
assign an mspace width - not just one that happens to be as large as
<mo>-</mo>.