GH,

Function EvaluateThis(myModelFormula As String, _
inRange As Double, myReplace As String) As Double
EvaluateThis = Application.Evaluate(Replace(myModelFormula, _
myReplace, CStr(inRange)))
End Function

This assumes that ModelFormula, LogCCoef, and LogBCoef are properly defined
named formulas.

Used like

=EvaluateThis(ModelFormula, B12, "x")

If you can't get it to work, I have a working sample workbook that I could
send you.

HTH,
Bernie
MS Excel MVP

wrote in message
oups.com...
I need some help creating a UDF that will evaluate the results of a
string that has been created using concatenate() that includes an
argument to be substituted in the resulting text string.

Concatenate results with the following text string:

(LogCCoef*LN(x))+LogBCoef

LogCCoef and LogBCoef are named excel formulas that return values. The
"x" is the placeholder for the argument that I would also pass to UDF
in the form a cell reference. The UDF would look like:

EvaluateThis(ModelFormula,b12)

Assuming:
ModelFormula (a named formula) is:
=concatenate((LogCCoef*LN(x))+LogBCoef)
LogCCoef (a named formula) returns: -6.35
LogBCoef (a named formula) returns: 55.8
Value in cell b12 is: 400

The UDF would return 17.75 [(-6.35*ln(400))+55.8]
Could someone provide some guidance?

GH