Thanks for this article.
I had to share a big partition with ext4 fs from a netboot on my server and had only an old version of parted, not handling ext4. And no way to upgrade it on the reasd-only rescue system. I followed these instructions (except for the 'margin', 3% of 1 TB is much space !!!, I simply added about 1Mo, just in case...) and it worked perfectly.