Well, Line 6 is not 0, of course.
But Line 7 is 0, (in the mini worksheet, Distribution $X, Rolled over $X, therefore, Line 7: $X- $X = 0)
I think the key point is,
my total basis over the last 10+ year, let's say, it's $100k, and I only converted $50k, therefore, it is comparing to your total basis, not the particular lot that you converted, therefore your this conversion has no gains ($50k - $100K is less than 0, so no gain). Maybe that's how IRA calculates that, I read the Form 8606 Instructions, I don't think I entered any thing wrong.
Thanks for your input!
