I can see what the OP is talking about, not all bosses are created equally. Unfortunately, boss visual model size, targeted red circle size, and actual boss melee hitbox size don't always match up (rarely do actually). While this problem is 20 times worse in PvP (because getting into the melee hitbox of something constantly moving + latency = hell), it does rear its ugly head in PvE. You can have your rotation memorized and down to motor skill reaction completely, but that just makes your reaction time to changes faster... the trick is noticing when something's wrong.
I suppose there's multiple addons that tell you if you're out of range, not behind the target, etc., but the simple solution is have on your scrolling combat text (even the in-game one is acceptable). If something moves out of your range, you won't see white attack damage anymore (ironically, I've seen yellow attacks having slightly larger range than auto-attacks as a feral druid... heck you can have the "you're facing the wrong way" error from auto-attacks and still hit with direction yellow attacks to some degree). There's also some bosses where you can Shred but still be parried (happens on Sindy a lot). The boss melee hitbox is rather dynamic between every encounter and boss... eventually you'll just get the knack for an encounter and adjust your positioning accordingly. Beyond that, the faster you recognize you're not in the correct melee position, the faster you can fix the situation. I'm sorry to say, but since you're a feral kitty, you'll have the toughest time positioning yourself since we have the only remaining ability that has positional requirement (other than Backstab, but it's not an issue for most rogues).