Pretty much immediately, as soon as you pass the LSO platform and are inline with the round down, start your turn (basing this off of your head position, not anything else on the aircraft). If you can see the back of the ship you are about to be too long. Also, should be about 1.2nm abeam as it is optimum.
on launch, make your 20 deg. clearing turn, then turn back to the BRC, climb to 600ft and at 1.0nm you can turn back into the downwind as long as there is no other traffic there.