A question on Hair Bleaching. Please Help?

Ive recently bleached my hair few times to go platinum blonde. However the roots are still lighter than the rest. The roots and a few cm from them is white..where the rest is a pale yellow colour which I like. I have already toned it and dont wanna do it again as it left blue in my hair on the top. My question is im going to bleach my hair from the midlengths to the ends soon but how do i do it do I dont overlap the lighter already over bleached area at the top? Is this called spot bleaching? Should I do it in sections to get it more even? Pls help thankyou.