What vol/percentage of bleach should I use?

my roots are a light-auburny-dark-blonde. I payed no attention before (fail) and got a 60% bleach peroxide which totally buggered up my hair, when I do it again (I don't feel complete not being totally blonde) in 5 months time, I want to know the right percentage to use (I'll try to go to the salon whenever possible, but being a student it's hard to cover the cost!).