you know the formula for RPI? its 25% home/away weighted win loss, 50% opponent win/loss, 25% opponent opponent win/loss. somewhere in there you have to exclude the game against yourself or something... like in the opponent opponent win loss maybe, because you are every opponent's opponent?
anyway, SOS is exactly the same as the RPI formula, minus the first bit - your own weighted record. in the RPI formula, the record of your opponents is NOT weighted by home/away, same with the opponent's opponent part. so the only bit of RPI that included home/away weighting, is the only bit that is not part of SOS. as you can guess, by SOS not including any home/away weighting, SOS is not really that fantastic of a measure of true strength of schedule.