If you need a second brood box, which you probably do to enable your bees to survive your winters, you should add that before adding any other boxes. Add boxes one at a time, or you will cause problems for the bees.
As to when you should add another box, it doesn't matter if that box is a brood box, the Flow super or even a traditional super. The rules are the same and very consistent. Add a box when all of the below are true:
1. All frames have fully drawn comb, and
2. That drawn comb is 80% full of brood, honey or pollen, and
3. Every frame is well-covered with bees
When all of those are true, you have enough bees to defend, use and heat the new space. If you add a box too early, they won't use it, and they may have problems with wax moths, chilled/chalk brood and other pests.
Hope that helps.