[texhax] Space factor as dimen?

[Joel C. Salomon]

Donald Arseneau wrote:
>> I found the word I needed---space factor.  To rephrase the question
>> above: can I supply a space factor to a macro expecting a dimen?
> 
> No.  
> 
> You probably have to do something like 
> 
> \newlength\myspc
> \settowidth\myspc{\spacefactor3000{ }}
> 
> and then use \myspc as the dimension argument.  That won't track 
> font size changes though.

I found most of what I was looking for, except for the bit doesn’t work:

\newlength{\issp}	% default inter-sentence space
\setlength{\issp}{\fontdimen2\font}	% inter-word space
\addtolength{\issp}{\fontdimen7\font}	% extra space

\newlength{\isst}	% inter-sentence stretch
\setlength{\isst}{\fontdimen3\font}	% inter-word stretch
\multiply\isst3		% mimic \spacefactor 3000

\newlength{\issh}	% inter-sentence shrink
\setlength{\issh}{\fontdimen4\font}	% inter-word shrink
\divide\isst3		% mimic \spacefactor 3000

\newlength{\intersentencespace}
%\setlength{\intersentencespace}{\issp plus \isst minus \issh}

The funny thing is, this /does/ work:

\newlength{\iwp}
\setlength{\iwp}{\fontdimen2\font %
	plus \fontdimen3\font minus \fontdimen4\font}

(In fact I could see the difference between using ‘{\iwp}’ as an
argument to \newtheoremstyle and using ‘{ }’, because the stretch was
applied.)

How can I set a rubber length the way I’m trying to?

—Joel Salomon