For superior block on heavy startup (such as Conqueror) the timing is slightly before the regular parry timing. I would imagine the same holds true for superior block on light attacks so give that a try. You really need to practice it, it's only a 2 frame window on console. Also take a look at a slow motion replay of a Warlord light attack, the shield should glow gold when superior block is active. That might help you determine exactly when the timing is.