How does power guard work exactly?

I understand that for non-boss monsters, you only take 60% of the damage dealt to you touch wise, and the knockback on you is lessened too. And for bosses, you take 80% of touch damage, is that correct? I can't figure out why I can't take a horntail tail touch with 30k hp and pg on. There was no poison and pg was definitely on and I had 30k hp 100%. I rushed into the tail from the right and died just like that. Is there something about pg that I'm not aware about? DKs can cross with max achilles no problem all the time, and it's only 15% reduction, compared to pg's 20%. I don't get it.