Too light or too quick?

Hi, I had a not-disasterous but not-optimal experience with doing my own highlights. I have for a couple of years done my highlights with a Miss Clairol violet-base color in Ash Blond and level 20 developer. (I have medium-dark blond hair, with orangey undertones, and the violet makes them come out nice and neutral, very subtle and natural looking.).
This time, I tried to add another level lighter highlight with level-30 developer and the next shade lighter in violet-base ash blond. Now I have some brassy tones on top. Did I not leave the color on long enough, or was the level 30 too much of a jump? Is it best to re-do the same thing & leave on longer, or use a deposit-only color to cool it out? FYI, I am the only one who thinks it is too red - my friends & fiance think it looks nice.