Having flown darkness since 1994, at different locations and different systems (paired up, separated etc) I have come to the conclusion that you don't need to pair up until at least February 1st, then darken them for a period of 6-8 weeks, depending when your young bird race programme finishes, if its October give them 8 weeks darkness and most importantly you don't have to darken them for 16 hours a day. Mine are only darken for 13 hours a day. Also I prefer the loft to be as dark as possible, without obstructing your ventilation. ;)