flushing is running copious amounts of water through the medium. Usually three times the volume of the vessel. This is done to try and remove excess nutrients from the medium. Often this is done at the end of bloom cycle.

5th week sounds good, especially if you are going with CFLs. You need to have those lights very close to the bud sites so a plant starting at 12"+ may end up at 24"+ and that's a lot of area to close in on with CFL lamps.