Yes, your equation is correct. Most people fail to take into accound that each son and daughter has brothers and sisters, but they are not their OWN brother or sister, but you caught onto that in your formula. Karma for you and jnuts.
Thanks slobberknocker. Yeah I'm a fantasy dork. I just got talked into DMing another D&D game twice a month. I'll have people driving to my place from both Dallas (4 hour drive) and Houston (1.5 hour drive), for the game. Hmmm... how did Royce talk me into this... I haven't done this in over 2 years.