Reformat tabular test data ending with a lengthy description onto multiple tabulated lines

[Graham Lawrence]

 
Here is my test data
Example 1     Of medium length     Enough words in description 
to need one reformat
Example 2     Very short one       Needs no reformat
Example 3     Extremely long one   This description is long 
enough and drivels on and on pointlessly for no other purpose 
than to trigger multiple reformatted lines.

Here are the :s commands which produced the best results so far and show exactly what I'm trying to achieve
:'a,'b-1s/.\{66,}/\=substitute(submatch(0),'\(.\{35}\)\(.\{,30}\) \(\(.\{,30}\)[ \n]\)\+','\1\2\r@@\1\3','')/g
:'a,'b-1s/^@@\(.\{35}\)/\=substitute(submatch(1),'.',' ','g')
Here is what they do to my test data
Example 1     Of medium length     Enough words in description to
                                   need one reformat
Example 2     Very short one       Needs no reformat
Example 3     Extremely long one   This description is long
                                   multiple reformatted lines.

So, it loses the interior lines of a multiple reformat.  To get 
those I revisualized the structure of these lines thus:
\(.\{35}\)\(.\{,30}\)\( .\{,30}\)\+
and that all I really needed to do is replace that leading 
space in each of the repeated third fields with \r@@\1, ie 
that my first :s should be something like
:'a,'b-1s/.\{66,}/\=substitute(submatch(0),'\(.\{35}\)\(.\{,30}\)\( .\{,30}\)\+', "\=substitute(submatch(3), '^ ', '\r@@\1', '')", 'g')
which produced this output
=substitute(submatch(3), '^ ', '
@@ ', '')
Example 2     Very short one       Needs no reformat
=substitute(submatch(3), '^ ', '
@@ ', '')=substitute(submatch(3), '^ ', '
@@ ', '')
which I read as the second \=expression is not being treated 
as such, but no variations like doubling the \ to cater for 
the enclosing "s makes any difference. Advice as to how to 
proceed, please.

--

Graham Lawrence