It's a pretty sure bet that if the dropouts are equidistant from the bike's centerline and the wheel closer to one stay than the other that one of the c-stays is a shade longer than the other. This would be VERY difficult to measure properly and accurately as it takes only a very small difference to make the wheel a good ways off center.
The way to make things are work out well is to slightly file the dropout slot so that the axle can slip just a bit further back on one side and then land in the center of the c-stays. It will take just a few file strokes and the wheel will move over. A little goes a LONG way so go slow.
I would strongly consider adding the s-stays before you do this. Heat will make things move around some and you will likely need to do the same thing (file the slot to get the wheel to center) between the s-stays so if you wait you can do it all at the same time. Less tail chasing this way.
Have fun.
dave