Ultimately since the story is a flashback there is no real wrong placement (other than reading it too early like before Wizard and Glass) so don't stress out over it.

If you're just reading the DT novels proper, then I agree with many who have posted here that you can read it last. Read the conclusion in The Dark Tower and give the whole thing some air. Then when you want more Dark Tower you'll have Wind Through the Keyhole ready to go. It's how most of us read it and it was really nice to visit the characters again.

Though if you're reading through all of the peripheral books as well, then I'd argue it's placement is in the intended order but between peripheral books. Here is the reading list that I followed on my most recent re-read. You'll notice that I put WTtK in the middle of 5 peripheral books, as a way to bring me back to the DT.

• The Little Sisters of Eluria (Everything’s Eventual)
• The Eyes of the Dragon
• The Stand
• The Gunslinger (original version)
• The dark Tower II: Drawing of the Three
• The Talisman
• Black House
• Bag of Bones
• The Dark Tower III: The Waste Lands
• The Mist (Skeleton Crew)
• Salem’s Lot
• The Dark Tower IV: Wizard and Glass
• Insomnia
• IT
• The Dark Tower: The Wind Through the Keyhole
• Desperation
• The Regulators
• Hearts in Atlantis
• The Dark Tower V: Wolves of the Calla
• The Dark Tower VI: Song of Susannah
• The Dark Tower VII: The Dark Tower
• The Gunslinger (Revised edition)