msgbox for negative answer and how to determine the number of significant digits

I have done this whole VBA project for several weeks(If you read my previous posts) and for some reason when I get a negative answer for my "re" when re=((4 * rate) / (pi * d * u)), I don't get a msgbox to appear telling the user there is an error. Also I was told to Format the Reynolds number to display a proper number of significant digits (see the VBA Format function). The VBA code does not need to decide how many digits to display; you should pick a reasonable number based on the data given and have the code implement that formatting. Any help I get will be very much appreciated.
Brandon

Private Sub reynolds_number_Click()
Dim flow As String, rate As Single, re As Single, p As Single, d As Single, u As Single, pi As Single

flow = InputBox("Select which flow rate is being used", "Flow Rate", "kg/sec, m/sec, m³/sec")
Select Case flow
    Case "kg/sec", "m/sec", "m³/sec"
        'Do nothing
    Case Else
        MsgBox "Invalid flow rate specified!", 16, "Invalid Units"
    Exit Sub
End Select

If flow = "kg/sec" Then
    rate = InputBox("Input a numeric value for the flow rate", "Flow Rate", "number")
    u = InputBox("What is the viscosity?", "Viscosity", "kg/m·sec")
    d = InputBox("What is the diameter of the inside of the tube?", "Diameter", " in meters")
Else
    rate = InputBox("Input a numeric value for the flow rate", "Flow Rate", "number")
    u = InputBox("What is the viscosity?", "Viscosity", "kg/m·sec")
    p = InputBox("What is the density?", "Density", "kg/m³")
    d = InputBox("What is the diameter of the inside of the tube?", "Diameter", " in meters")
End If


pi = 4 * Atn(1)
re = ((4 * rate) / (pi * d * u))
re = ((d * rate * p) / u)
re = ((4 * rate * p) / (pi * d * u))
Select Case (re)
Case Is < 0
    MsgBox "You have inputed a incorrect number causing the answer to be negative", 16, "Error"
    Exit Sub
Case Is < 2100
    MsgBox "The flow is laminar", , "Fluid Flow"
Case Is > 2100
    MsgBox "The flow is in transition", , "Fluid Flow"
Case Is < 6000
    MsgBox "The flow is in transition", , "Fluid Flow"
Case Else
    MsgBox "The flow is turbulent", , "Fluid Flow"
End Select
    

If flow = "kg/sec" Then
    MsgBox "The Reynolds Number is " & ((4 * rate) / (pi * d * u)), , "Reynolds number if volumetric flow rate was specified"
ElseIf flow = "m/sec" Then
    MsgBox "The Reynolds Number is " & ((d * rate * p) / u), , "Reynolds number if velocity specified"
ElseIf flow = "m³/sec" Then
    MsgBox "The Reynolds Number is " & ((4 * rate * p) / (pi * d * u)), , "Reynolds number using volumetric flow rate"
End If



End Sub

Thank you very much. You do not know how much this means to me. I have been straining for a long time. Do you have any idea what I should do for the sig fig part?