I want to dye my dark brown hair to a dark to medium ash blonde colour, plz help.?

[Liz26]

I have very dark brown hair, almost black, and I would like to dye my hair to a medium to dark ash blonde colour. Should I use 30% peroxide with a medium ash blonde hair dye, or should I use 20 % with a very light ash blonde colour?

Please help, thankyou in advance.
Liz