Stop text box from auto refreshing in VBA

I'm currently encountering a problem. I have set up an if statement on a text box - if the number is smaller than 2.4, and larger than 6 then return an error.

However, if a user wishes to input 2.8, it will throw the error when 2 is entered, and when the '.' is entered.

Here is my code:

Private Sub txtHeight_Change()
HeightNumber = CDbl(Val(Me.txtHeight.Value))

If Not Me.txtHeight = "" Then
    If HeightNumber >= 2.4 And HeightNumber <= 6 Then

    Else
        MsgBox ("You have entered an incorrect number. Please enter a number between 2.4m and 6m.")


End If
End If
totcost = (HeightNumber * Width1) * (painttype + undercost)
            TotalCost.Value = totcost
End Sub

Thanks Olly, but now it isn't updating at all in my "Total" text box

I have changed it to AutoRefresh instead of Change. It now works - thanks.