Rolls a number of dice with a number of sides, and adds a modifier. In standard gamer dice notation (SGDN), a roll of 3d8+2 looks like:    RollDice(8, 3, 2)
Single-die rolls are oh-so-simple: a d100 is RollDice(100).

Function RollDice(DiceSides As Integer, Optional DiceNum As Integer, Optional DiceMod As Integer)
    If IsMissing(DiceNum) Or DiceNum = 0 Then DiceNum = 1
    If IsMissing(DiceMod) Then DiceMod = 0
    If DiceNum < 1 Or DiceSides < 2 Then
        RollDice = 0
        GoTo RollDiceEnd
    End If
    RollDice = (DiceNum * Int(Rnd * DiceSides + 1)) + DiceMod
RollDiceEnd:

End Function