This won't help you now, sorry...but for next time, this is how we do it.
Any time there's something that print double-sided, we always send it centered on the stock...but we also print a thin colored line several inches ahead of where the actual image starts and the same colored line at the end with the same spacing. Also, we mark a place on the stock that matches some piece on our machine. Then, the first side is dry and reloaded, we use the distance between where the image started and our mark to make an educated guess for starting position, send the job and if the two colored lines aren't printed basically on top of on another, we cancel the job and try again.
It wastes some stock, but our stuff usually hits right on.