Aloha klizark007,
I haven't heard Master Lin speak to this other than to not sleep on your stomach. Let do of taking the "Spine Straight" literally. There is a natural curve to your spine, and something that would support that structure would be useful. If you're looking for specifics on pillow height, fill, or texture you'll need to check elsewhere and it sounds like you have.
Sleep like a baby, immovable like and mountain and still like an ocean without waves...
Much Love, Shawn
