> calculation really seems to be dependent on PHYSICAL 
> layout as well as logical dependency.
>

Definitely.  I once took a sheet from a client which was laid out to 
look nice when it was printed.  This resulted in a complete mess of 
linked cells all over the sheet.  It involved two circular references, 
one nested inside the other and took 2 minutes to recalculate.  I 
physically reorganised it so that each cell only depended on the cells 
to their top/left, apart from the two cells containing the circular 
ref.  This alone reduced the calculation time to 20 seconds.  It was 
then very easy to identify calculations that were inside the circular 
refs, but were not really needed there.  Stripping these out made it 
possible to remove the internal circular ref entirely (replacing it 
with a set of straight-through calculations).  The result was a much 
neater set of calulations which took 3 seconds to calculate (which 
included four iterations on average).

We were also then free to redesign the report, which was no longer 
constrained by the need to include all the calculations - we could just 
pick and choose parts of the calculations to display on a separate, 
linked, sheet.

Good luck

 Stephen Bullen
 Microsoft MVP - Excel
 http://ourworld.compuserve.com/homepages/Stephen_Bullen