Изменить возвращаемое значение в поле, чтобы считал процент при двух значений цен

Function CalculateHistoricalIncrease( _
                ByVal vdblOldAmount, ByVal vintOldCurrID, ByVal vdtmOldStartDate, ByVal vdtmOldEndDate, _
                ByVal vdblNewAmount, ByVal vintNewCurrID, ByVal vdtmNewStartDate, ByVal vdtmNewEndDate, _
                ByVal vintConvertToCurrID, ByRef robjCurrConv)
 
 
 
    Dim dblConvOldAmount      '::: Converted old amount
    Dim dblConvNewAmount      '::: Converted new amount
    Dim dblIncrease           '::: Increase in %
 
    '::: If a currency to convert to is specified, use it; otherwise use the currency of
    '::: the newer record.
    If IsNull(vintConvertToCurrID) Then
      robjCurrConv.CurrIDOut = vintNewCurrID
    Else
      robjCurrConv.CurrIDOut = vintConvertToCurrID
    End If
 
    '::: Calculate converted old amount. Use end date of old record. If the end
    '::: date of the old record is NULL, use current date.
    If IsNull(vdtmOldEndDate) Then
      vdtmOldEndDate = Date
    End If
    robjCurrConv.CurrIdIn = vintOldCurrID
    dblConvOldAmount = robjCurrConv.Convert(vdblOldAmount, vdtmOldEndDate)
 
    '::: Calculate converted new amount. Use start date of new record.
    robjCurrConv.CurrIdIn = vintNewCurrID
    dblConvNewAmount = robjCurrConv.Convert(vdblNewAmount, vdtmNewStartDate)
 
    '::: If either the old or new salary is NULL (that is: not defined or
    '::: cannot be converted) then the result of this function is NULL.
    dblIncrease = Null
    If ((Not IsNull(dblConvOldAmount)) And (Not IsNull(dblConvNewAmount))) Then
 
      '::: Double-check type
      dblConvNewAmount = CDbl(dblConvNewAmount)
      dblConvOldAmount = CDbl(dblConvOldAmount)
 
      '::: Prevent division by zero
      If dblConvOldAmount = 0 Then
        dblIncrease = Null
      Else
        dblIncrease = ((dblConvNewAmount / dblConvOldAmount) -1) * 100
      End If
 
    End If
 
    '::: Set return value
    CalculateHistoricalIncrease = dblIncrease
 
  End Function

 Parameters:
   - vdblOldAmount           Money amount of old salary amount
   - vdblOldCurrID           Currency of old salary amount
   - vdblOldStartDate        Start date of old salary amount
   - vdblOldEndDate          End date of old salary amount
   - vdblNewAmount           Money amount of old salary amount
   - vdblNewCurrID           Currency of old salary amount
   - vdblNewStartDate        Start date of old salary amount
   - vdblNewEndDate          End date of old salary amount
   - vintConvertToCurrID     Curreny to convert amounts to
   - robjCurrConv            Reference to a ready-to-use CurrencyConverter object