I’m in the middle of an exterior insulation retrofit for climate zones 7+8, and installed a large innie window with nailing flange tied to the new sheathing and WRB.  I followed best practices for flashing and sealing.  However, for the remainder of the windows, I’d like to go without the nailing fin which, while convenient for sealing and flashing, creates a ledge on the exterior face of the WRB/sheathing.  This little detail complicates subsequent flush fitting of exterior wood blocking around the frame used for supporting exterior PVC sill and jam extensions. 

So the question is, in the absence of a nailing fin, what are best practices for *exterior* sealing the typical gaps (1/4″ to 1/2″) between PVC window frame and rough openings and tie in with WRB?  I’m thinking backer rod + PU sealant smoothed flush, then peel and stick over top when cured.   Please point to GBA or FHB articles if they exist.