Excel persistent evaluate if condition is met - excel

Question

I am trying to evaluate an expression only if a certain condition is met. The reason I am doing this is to allow the user to "lock" a value so that changes to other variables in the formula have no effect anymore.
I tried using the function below which works great until I close the sheet and open it again.
I already tried to use an additional cell passed as parameter to copy the value to it when it's not locked and copy it back if so however excel does not allow other cell modifications within a function.
Is there any way to achieve this functionality?
Function EvaluateIf(expression As String, condition As Boolean) As Variant
Application.Volatile
Dim myText As String
Dim myVal As Variant
If condition Then
myVal = Application.Evaluate(expression)
Else
myText = Application.Caller.Text
If IsNumeric(myText) Then
myVal = Val(myText)
Else
myVal = myText
End If
End If
EvaluateIf = myVal
End Function
EDIT1:
I need to apply this function onto multiple cells so I cannot hard code the cells
EDIT2:
I currently call the function like this in excel:
=EvaluateIf(N$7*IF(ISBLANK(P$7);1;P$7)*IF(ISBLANK(R$7);1;R$7);NOT(V$7))

Answer 1

Try this out - normal cautions apply to using this method to skirt around the restrictions applied to the use of UDF when called from a worksheet.
Function EvaluateIf(expression, condition As Boolean, backup As Range) As Variant
Dim myText As String
Dim myVal As Variant
Dim bak
bak = backup.Value
If condition Then
myVal = expression
If myVal <> bak Then 'update cached value?
Application.Evaluate "SetBackup(""" & backup.Parent.Name & """,""" & _
backup.Address & """,""" & myVal & """)"
End If
Else
myVal = bak
End If
EvaluateIf = myVal
End Function
Sub SetBackup(ws As String, addr As String, v)
Application.Calculation = xlCalculationManual 'avoid infinite loop!
ThisWorkbook.Sheets(ws).Range(addr).Value = v
Application.Calculation = xlCalculationAutomatic
End Sub