One Digit Checksum

I've made my first attempt at making a number checksum creator/validation. After going round in circles, I've given up

Can some kind person please tell me where I've gone wrong with the below?

'140731
Option Explicit
Option Private Module

Private Const mbytcMod As Byte = 11

Public Sub TestMod11Checksum()
    Dim avarWeights() As Variant
    Dim strRegNo As String
    Dim bytCheckDigit As Byte
    Dim blnIsValidChecksum As Boolean

    avarWeights = Array(1, 4, 3, 7, 5, 8, 6, 9, 10)

    Debug.Print "Create random 8 digit string"
    strRegNo = Int(Rnd * 100000000#)
    Debug.Print "Random no. = " & strRegNo

    Debug.Print "Attempt calc of checksum digit for random no."
    bytCheckDigit = fn_bytCalcChecksumDigitMod11(strRegNo, avarWeights)
    Debug.Print "Check digit = " & bytCheckDigit

    Debug.Print "To test checksum validation works, we will add the check digit to the end of the 8 digit no. then run this through the function"
    strRegNo = strRegNo & bytCheckDigit
    Debug.Print "Random no. is now " & strRegNo

    blnIsValidChecksum = fn_blnValidChecksumMod11(strRegNo, avarWeights)

    Debug.Print "The checksum validation returned: " & blnIsValidChecksum
End Sub

Public Function fn_bytCalcChecksumDigitMod11(ByVal strChecksum As String, ByVal avarWeightFactors As Variant) As Byte
'provide the full weighting array - i.e. include the weight for the checksum digit
    Dim bytPosition     As Byte
    Dim intSum          As Integer
    Dim bytCurrent      As Byte
    Dim blnValid        As Boolean
    Dim bytWeight       As Byte
    Dim bytArrOptFix    As Byte
    Dim bytRemainder    As Byte
    Dim intCheckDigit   As Integer

    If Not IsNumeric(strChecksum) Then
        Exit Function
    End If

    If Not IsArray(avarWeightFactors) Then
        Exit Function
    End If

    If Not Len(strChecksum) = UBound(avarWeightFactors) - LBound(avarWeightFactors) Then
        Exit Function
    End If

    'auto handle base0 and base1 arrays
    bytArrOptFix = 1 - LBound(avarWeightFactors)

    'Multiply each digit by its weighting factor
    For bytPosition = 1 To Len(strChecksum) - 1

        'get the value of the current digit
        bytCurrent = CInt(Mid(strChecksum, bytPosition, 1))

        'get the value of the current weight factor
        bytWeight = avarWeightFactors(bytPosition - bytArrOptFix)

        'add the multiplied value to the total sum
        intSum = intSum + (bytWeight * bytCurrent)
    Next bytPosition



    'I'm dubious about the next few lines of code
    Select Case intSum
    Case Is < mbytcMod
        intCheckDigit = mbytcMod - intSum
    Case Is = mbytcMod
        intCheckDigit = 0
    Case Is > mbytcMod
        bytRemainder = intSum Mod mbytcMod

        If bytRemainder = 0 Then
            intCheckDigit = 0
        ElseIf bytRemainder > 0 Then
            intCheckDigit = ((bytRemainder + 1) * mbytcMod) - intSum
        Else
            Debug.Assert False
            'shouldn't reach here?
        End If
    End Select

    fn_bytCalcChecksumDigitMod11 = intCheckDigit
End Function

Public Function fn_blnValidChecksumMod11(ByVal strChecksum As String, ByVal avarWeightFactors As Variant) As Boolean
    Dim bytPosition     As Byte
    Dim intSum          As Integer
    Dim bytCurrent      As Byte
    Dim blnValid        As Boolean
    Dim bytWeight       As Byte
    Dim bytArrOptFix    As Byte

    If Not IsNumeric(strChecksum) Then
        Exit Function
    End If

    If Not IsArray(avarWeightFactors) Then
        Exit Function
    End If

    If Not Len(strChecksum) = UBound(avarWeightFactors) - LBound(avarWeightFactors) + 1 Then
        Exit Function
    End If

    'auto handle base0 and base1 arrays
    bytArrOptFix = 1 - LBound(avarWeightFactors)

    'Multiply each digit by its weighting factor
    For bytPosition = 1 To Len(strChecksum)

        'get the value of the current digit
        bytCurrent = CInt(Mid(strChecksum, bytPosition, 1))

        'get the value of the current weight factor
        bytWeight = avarWeightFactors(bytPosition - bytArrOptFix)

        'add the multiplied value to the total sum
        intSum = intSum + (bytWeight * bytCurrent)
    Next bytPosition

    'If total is exactly divisible by mod factor then checksum is valid
    blnValid = (intSum Mod mbytcMod = 0)

    fn_blnValidChecksumMod11 = blnValid
End Function