Good quality bleach vs. developer

The developer level just means how fast it'll work. So, if your bleach powder can only lift 4 levels for example, a 40 vol will make it lift those 4 levels faster, but not past those 4 levels. If your bleach can lift 8-9 levels the same applies, 40 vol will do it faster 20 or 30 vol will make it a more manageable time. The darker your hair is the more likely it is you'll need a higher volume as it takes longer to lift through the colours (red to yellow) and very resistant hair will need 30vol. Never use 40 vol on the scalp though. I personally don't think it should be sold at all as it can cause horrendous chemical burns and can destroy hair easily. This is why I emphasis a good bleach powder. If it can lift 8-9 levels it will brag about it on the package!

It's always better to use as low volume as you can get away with because you can find the place you started to bleach is already at the stage it needs to be washed off but you haven't even finished yet! So someone with light blonde hair wouldn't be advised to use 30 vol as it's going to work far too quickly on their hair. As you have Asian hair, you will likely need to go with 30 vol, but work as quickly as you can.

I wouldn't advise bleaching twice if it can be done once. The problem is, you should never bleach your hair more than once a month. You need to make sure you haven't done too much damage before bleaching again to add to it. If you don't get the desired result the first time, you need to wait and of course your roots grow. You'll always have roots that are one bleaching behind the rest and so will never match! You want to be able to do your roots in the future and have them match the lengths and not have to bleach them twice.