If you cannot find the documentation of when your statutory road association was formed, I think the best thing to do to avoid confusion in the future would be to go through the process of starting an association as if it has never been done before. That would give you documentation to have on record, and would also allow you to make sure your bylaws are worded to include any new subdivision. The only down side (aside from a little bit of extra work for the initial Notices,) would be that if you have any outstanding Notices of Claim, the new association probably would not be able to claim them. You'd have to start fresh. If everyone is paid up, that won't be a problem.
As for how to calculate dues when adding new subdivisions, everyone who uses a road should be contributing to its maintenance. So the people in the new subdivision need to pay for maintenance of the original road which is used to get to the subdivision. But if people on the original road have no right to use the roads in the new subdivision, and/or have no reason to use them even if they were allowed to, it may make sense to have the subdivision form their own association to maintain their own roads, in addition to paying to use the original road. One way I've seen that handled is by having everyone in the new subdivision just make one payment to their own association, and then their own association makes a lump sum payment to the original association to cover everyone's use of the original road.
One thing that doesn't sit right with me is when a town approves a subdivision off of a road the town does not maintain, forcing those who depend on the original road for access to suddenly see increased use of their section of road, without their having any say in the matter. There are cases where the existing road was not built to withstand the extra traffic a subdivision will bring in, but the town doesn't seem to consider that. The town just sees that more development will bring in more tax revenue, and since the development is on a private road, the town's expenses for road maintenance will not increase. So they will reap the benefit of increased tax revenue that can be spent elsewhere in town.
I've also seen situations where people in a new subdivision off of a private road decide to form a road association and want everyone on the original road to chip in to help maintain the new road. Your bylaws could clarify what sections of road your association maintains, and make it clear that anyone whose lot benefits from use of those sections of road must be a paying member of your association. Again, starting with an initial Notice issued by a Notary as if you are forming a new association would allow you to make sure things are set up clearly and according to law.