Excel 97 Visual Basic

I am trying to create a simple form using the Visual Basic Editor in Excel 97.
The idea of this form is to automate a simple Goal Seek, with an option to also create a set of Scenario’s using the Goal Seek data.
The Goal Seek is designed to forecast how much pay rise the firm can afford, with a limit that the Annual wages bill must not exceed 3.85% of the Annual Turnover.
If I run the form from within the VB Editor it works fine, but as soon as I place a Command Button on the actual worksheet to run the form I get the following Error.
Run-time error ‘1004’
Goal Seek method of range class failed

The highlighted code is:
Range("I19").GoalSeek Goal:=myNum, ChangingCell:=Range("I17")

The Command Button code is : usfGoal_Seek.Show.
Typical Data used:
Proposed Pay increase             = 3.50%
Annual Wages Bill                    = £183,419.56
Estimated Annual Turnover       = £4,603,012.99

Private Sub chkYes_Click()  'Create Scenario option
  Clear                     'Sub Routine to clear Text Boxes and Setfocus
End Sub

Private Sub cmdAdd_Click()  'New Goal Seek Button
  Clear                     'Sub Routine to clear Text Boxes and Setfocus
  cmdSeek.Visible = True    'Show Goal Seek Button
  cmdView.Visible = False   'Hide View Scenario Button
  cmdAdd.Visible = False    'Hide New Goal Seek Button
End Sub

Private Sub cmdSeek_Click()         'Goal Seek Button
If txtTitle.Text = "" Then          'Check for Text in Title Box
  Dim Msg1, Style1, Title1          'Error Message
  Msg1 = "PLEASE ENTER A TITLE !"
  Style1 = vbCritical
  Title1 = "CANNOT GOAL SEEK"
  Response = MsgBox(Msg1, Style1, Title1)
  txtTitle.SetFocus                  'Return Focus to Title Box
ElseIf txtTurnover.Text = "" Then    'Check for Turnover amount
  Dim Msg2, Style2, Title2           'Error Message
  Msg2 = "PLEASE ENTER THE TURNOVER AMOUNT !"
  Style2 = vbCritical
  Title2 = "CANNOT GOAL SEEK"
  Response = MsgBox(Msg2, Style2, Title2)
  txtTurnover.SetFocus               'Return Focus to Turnover Box
Else
  cmdAdd.Visible = True              'Show New Goal Seek Button
  cmdView.Visible = True             'Show View Scenario Button
 If chkYes.Value = True Then         'If Check Box is Checked
   myNum = txtTurnover.Text          'Assign Turnover amount to "myNum"
 
  'Range("I19").GoalSeek Goal:=myNum is Turnover amount'
  'ChangingCell:=Range("I17") is Percentage amount'
 
   Range("I19").GoalSeek Goal:=myNum, ChangingCell:=Range("I17")
   txtPercent.Text = Range("i17")    'Percentage
   txtBill.Text = Range("i18")       'Wages Bill
 
   'Add the results to the Scenarios sheet'
 
   ActiveSheet.Scenarios.Add Name:=txtTitle.Text, ChangingCells:=Range("d12,d13"), Values _
        :=Array(txtPercent.Text, txtBill.Text), Comment:="Created by K V Jones", _
        Locked:=True, Hidden:=False
   cmdView.Visible = True            'Show View Scenarios Button
  Else
   myNum = txtTurnover.Text          'Assign Turnover amount to "myNum"
   
   'Range("I19").GoalSeek Goal:=myNum is Turnover amount'
   'ChangingCell:=Range("I17") is Percentage amount'
 
   Range("I19").GoalSeek Goal:=myNum, ChangingCell:=Range("I17")
   txtPercent.Text = Range("i17")    'Percentage
   txtBill.Text = Range("i18")       'Wages Bill
  End If
End If
End Sub
Private Sub UserForm_Activate()
  Worksheets("Negotiation Scenario").Activate
  Clear                     'Sub Routine to clear Text Boxes and Setfocus
End Sub
Private Sub Clear()         'Sub Routine to clear Text Boxes and Setfocus
txtPercent.Text = ""
txtTurnover.Text = ""
txtBill.Text = ""
txtTitle.Text = ""
txtTitle.SetFocus
End Sub