I've asked on the SABRE forums before, and there was no consensus on 60 or 70, so I used my common sense of the roundabout being a single circulatory carriageway, therefore it's 60. If you want to settle this, I suggest again to make a discussion on the community forums, but I can't imagine it being correct for NSL roundabouts to be 60 or 70 simply depending on their connecting roads.