i think you are all trying to be exact. we breed our young birds in jan/feb and put them on the dark straight away, they are darkened from 5pm until 9am, if possible. they stay on the dark until the 1st July. They will be holding a full wing when the first race comes around. This year from 8 young bird club races we won 7 and won the fed 3 times as well. the main thing with the darkness is keeping them heathly, to many times i've heard people just shutting them in nothing more than a biscuit tin, no air, leading to respitory problems. You need to keep a good air flow, without causing a draft as this to will have a bad effect on the young birds. Remember the loft does not have to be pitch black, ours is dark enough that they roost up but you can go in there and see the birds easily. I think its more to do with the rest time rather than how dark the loft actually is.