Slaying Phantoms--Soul Drop question

This may have been asked before, but the search isn't helping me that much right now, so I apologize for it. I'd appreciate an answer or a pointer to the answer.

When you kill an invader or a summoned helper, what determines how many souls you get? Is it based on the vanquished phantom's level, or by how many loose souls they have?

On the phantom's level. For invaders, 50% of the required souls to level up. For hosts, 10% of the souls. For RSS and WSS phantoms, I'm not really sure... I always go here to check the SL of the player.
When I kill a host and he gives me +800,000 souls, the bastard must have been SL +600. I have a theory that the invasion system might be glitched, because these encounters with very high SLs are rare and don't happen all the time.

http://darksoulswiki.wikispaces.com/Stats

Ahhh. Thank you. I was wondering because I did a few rounds as a summon for both PVP and slaying Sif, and ended up with a fair whack of souls, but also noticed the soul drops varied. Someone might give 15k, another person might give 50k, and wondered if there was a feedback loop happening there.

Someone really needs to update the wiki with this info. Im pretty sure that phantoms get 25% of the souls needed to level up when a phantom dies.

Yeah me too, when I summon RSS I get ~22,000 souls when they die @ SL 120, instead of 45,000.

well, phantoms get 50% of what the host gets, so 25%
and it seems that it applies to RSS too if you're saying so