You should be using 20 volume since you are technically already a blonde. But if 30 volume's all you could find, you need to lessen the time it's your head than what it usually says. Remember that bleach is made especially for darker haired people who are trying to get to blonde. So you already being a blonde, need to lessen the time so you don't completely damage your hair.
Your hair will turn orange though. It always does. Because orange is the hair's natural base color for all humans. It's unavoidable. But it's good sign that's its orange. Because that means it has stripped all the color out of your hair. Which means you can proceed to dyeing your hair the desired color. However to achieve "white blonde" status, you need to buy a white blonde toner.
EDIT: You could use a product called Wella Silver Lady. It cancels out the yellows from bleach and turns hair white or any other shade of blonde. There's other toners that are shampoos too. You have do research on them because I'm not too sure which one is for white hair. But basically for toners you would use them as regular dye. For the shampoos they are about 5-10 minute leave ins.
Your hair will turn orange though. It always does. Because orange is the hair's natural base color for all humans. It's unavoidable. But it's good sign that's its orange. Because that means it has stripped all the color out of your hair. Which means you can proceed to dyeing your hair the desired color. However to achieve "white blonde" status, you need to buy a white blonde toner.
EDIT: You could use a product called Wella Silver Lady. It cancels out the yellows from bleach and turns hair white or any other shade of blonde. There's other toners that are shampoos too. You have do research on them because I'm not too sure which one is for white hair. But basically for toners you would use them as regular dye. For the shampoos they are about 5-10 minute leave ins.