Thanks Alifrog for the suggestion. That works just fine in a simple situation, but... my situation uses the result in the first filled Text Box as input parameters to another User Function and the propagation of "null" results in the problem also being propogated, as the User Function formula variables are all defined as numbers or strings and the required result must be a number.
Also, I'm doing all my calculations and data validation in VBA for Access and my User Functions have up to 6 or more parameters , so my VBA solution looks like this:
' Test if any parameters being used by the User Functions (a maths calculation and
' an imbeded formating procedure) are "null"
If IsNull(Me.A_Price) = True Or IsNull(Me.A_Price) = True Or IsNull(Me.A_Price) = True _
Or IsNull(Me.A_Price) = True Or IsNull(Me.Display_Currency_Symbol) = True _
Or IsNull(Me.Instrument_Decimal_Places) = True Then
' If a "Null" is found then insert a default value in the Text Box on the FormElse
Me.Tb_25_Milestone = 0
' If no input parameters report "Null" proceed with calculation and insert in Text Box on the FormEnd If
Me.Tb_25_Milestone = Format(CalcMilestone(0.25, Me.A_Price, Me.B_Price, Me.C_Price, _ Me.Long_Short), DecimalDisplayFormat(Me.Display_Currency_Symbol, Me.Instrument_Decimal_Places))
Thanks for the clue as to how to solve my problem.