Thanks for your reply. I'd like to try your first way, with the values
hardcoded in known cells. Can you be more specific in how the functions
would be layed out?

In my example, I have a function in the module:

public sub GetResults()
Dim size as double
size = Range( "$B$1" )

Dim results as ResultsObject
set results = ComplexCalculation( size )

Range( "$B$2" ) = results.Mass
Range( "$B$3" ) = results.Temperature
end sub

How would I set up the worksheet_calculate as you propose? I've tried
it several different ways but the Solver doesn't like it :(