40 vol to 20. Advice needed

How long are you leaving the bleach with 20vol on? It sounds like you can leave it on longer. You may need to go back up to 30vol. Stay away from 40vol from now on, you've been lucky so far, but you never know when it might cause really bad chemical burns!

So, the higher the volume of peroxide, the faster it works. Any volume has normally done everything it's going to do after around 45 mins to an hour. So, you can leave it on for anywhere up to 45 mins to be safe if there's no instructions on the bottle.

For now, you could use a bleach bath to do the roots again. There's a good tutorial on that on this forum in the tutorials section. Definitely don't be tempted to do a full bleach again.

If you don't want to do that, you could always even out the turquoise by adding a little yellow onto the lengths past the roots. Or, the more tricky one, is to add some purple to the roots to tone down the yellow. That is a much more tricky match to make and more difficult to do well.

Hopefully that helps 🙂