Was just wondering when and how does Rand decide that Shadar Logoth must be scooped and destroyed from randland to cleanse Saidin.

 

I think I totally read right by this part but it has been bugging me need some closure here

 

Thanks!

Well, he got the answer after his trip through the doorway in Tear. We don't know the exact content of the answer the Finns gave him, but it was detailed enough that Rand and Herid Fel could puzzle out the method from it.

 

Basically, when he drew saidin through the tube of saidar, he was bringing the taint into contact with Mashadar (Shadar Logoth's evil), which was an "opposite" evil to the Shadow. Once enough of the taint had made contact with Mashadar (which took a lot of saidin to accomplish), the opposing nature of Mashadar and the DO's taint caused them to start attracting and destroying each other.

 

I would assume the Healing Flinn did also gave him a clue, where Flinn said that he isolated the wound in Rands side from the rest of the body which made the two wounds (one from the shadow, one from Fain) fight eachother.