We are currently midway an exterior insulation project that includes 2″ of rockwool comfortboard 80 held in place with 3/4″ plywood furring strips.  the plan is to finish with hardie plank siding. 

with the exterior assembly totalling 2 3/4 from the sheathing to the front face of the furring strips i had planned my window buck to be 2 3/4 as well.  my problem now is a slight build out at the window bucks between fast flashing the RO,  window flange and another layer of fast flash to seal the window.  the complete window buck now stands about 1/4 to 3/8  proud of the adjacent furring strips.  the window trim itself can be corrected easily enough by shimming the exterior of each piece but that would leave my siding sitting slightly behind the trim where each lap sits on the furring strips. 

any thoughts or suggestions?  im debating adding some 1/4 ply nailed ontop of each furring strip to correct the gap.  definately making my window bucks 1/4 short in the future to allow for this.