... I've managed to do what I want without ANY cell formulas, and as a bonus, INDIRECT has opened a doorway to using conditional formatting by reference to another worksheet, normally prohibited by Excel.
If anyone wants a look at how I solved the original problem, and to see the C/F in action (small example file, 22Kb), mail me and I'll be happy to send it.
Now all I've got to do is apply the techniques to the REAL application file, when I get a few spare hours....
.. if anyone had any opinion of whether vba code would execute faster than conditional formatting in this case - I really want to get the calculation time of this application down to something more reasonable.
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