We built a plywood “Dudley box” for each rough opening. The Dudley box includes an integral plywood flange, and the flange is the same thickness as the vertical furring strips that will later be installed over the rigid foam. One key aspect of the Dudley box concept: the box is sized to match the window, not the rough opening.
So how do you get the house to be ultra tight? And ultra insulated? We have pretty much settled on one approach because it is robust and gets us consistently tight. Real tight. Ultra tight. We like advanced frame 2x6 construction (“BSI-030: Advanced Framing”) fully sheathed with plywood or OSB and wrapped with a fully-adhered membrane. Outboard of the membrane we with go with continuous insulation. Lots of continuous insulation. And we don’t care whose. They all work.