Many ways to do this . Start BB when stretch is over (at 1/4 rate) while still using the veg nutes at full rate. After 2-3 more weeks increase BB to (1/2 rate) and cut the veg nutes down to (1/2 rate) also. After 2-3 more weeks go to full rate with BB and 1/4 rate veg nutes. This is subject to what the plant is telling you !!  Happy growin