Everything has to do with your setup. Lights - If using CFL's or floro tubes then the canopy must remain as even as possible, if using HID the light has more penetration and depending on the wattage this depth will vary.
Plants - If you haven't topped & are just letting the plant run wild it will be less productive to tie the main cola down, unless you keep in down and let it grow horz. If topped just pull the main arms down (I don't think this is where you are). Another technique if not topped is to "Tip the top" (this is only used during the stretch after the flip) If you bend the newest growth over 90 degrees, tie it with a piece of string, wire whatever, and leave it that way for a couple of days then release, it will stunt the main stock stretch & encourage the lower arms to reach the same height as the main stock.
There are many ways to even out your canopy do some reading here and apply to your conditions.