I've asked myself the same question. And consider my nature, it's surprising I haven't posted the same question.
I did play with my hip positions for a while, since this will adjust the curvature of the lower back.
However, my interpretation has evolved to be a relaxed natural position. The straight spine and chin tucked in, I have changed to be the position my body would adopt if I was suspended by the crown of my head. In this way I loose the upper back slouch and the neck extends and the chin comes in slightly.
In this way my spine becomes long, long, longer.
You are perfection.
Iam2 
