I start at the roots, and blend toward the tips. I usually don't have enough to cover all the way to the tips, so I just blend until it's gone. Hubby has mentioned that the color gets darker toward the ends, and I've read that it will build up, but I'm happy with what I'm getting and don't see much color difference roots v ends myself. It blends smoothly, so color graduation is subtle enough to look natural so far (I've only done 4 applications). I really do love the stuff. Messy as hell, I don't love the smell, and the scent lingers, so it's there every time it gets wet... But the color doesn't bleed after two washes or so. I'd use a white towel with little concern, which I would never have done with box dye. Henna is a commitment. You can't just go back once you switch. But I've had a positive experience myself.
No poo is scary, but I've read great things. Let us know how the water only thing goes, I've not been brave enough to try. Low poo has definitely increased body and improved condition, but I still have to do at least a light lather on my temples about twice a week, and regular conditioning when I do. And I use whatever is handy, from fancy natural hippie bullshit to dollar store junk. I see very little difference honestly. It's the frequency and amount of shampoo used when washing that has made the difference for me. I went from a palm-full to maybe a quarter size drop massaged onto my temples only. I think my ends get no poo ever, truthfully, and my lack of split ends thanks me.
Sorry ramble. I've been excited about both of these discoveries.