If you are going to stay in the brown family, use a 30 vol developer, but stick with cool shades like Ash browns unless you want red highlights to come out. The red could be very pretty, but you need the strength of the 30 vol if you are lifting dark hair. 20 vol peroxide will not accomplish this and your hair will turn orangey. No matter what, if lightening dark hair use a minimum 30 vol developer and use a 40 vol if going more than 3 shades lighter. It is also helpful to buy a little sample tube of Red Corrector (they sell it at Sally's). When you mix your color and developer in the bottle just add 15 drops of the color corrector and shake/blend thoroughly. This will counter any tendency towards brasiness. Clairol Shimmer Lights is an excellent shampoo to counter brasiness and Sally's sells the generic. You can use it weekly.
What I do : use Medium Blonde color (Ion) with 2.5oz developer (Ion's is the best and most gentle, imo) using 30 vol or 40 vol.....40 vol seems to lighten faster.... Apply to roots and sit for 40 minutes. If I want more lightness, once hair is dried I will mix a small amount of powdered hair blech with 40 oz developer mixed in a bowl and painted on freehand with a mascara wand brush (also sold at Sally's). Most of the time the lighter blonde stays in just fine since i'm only putting the Medium Blonde base on the roots. My lighter points are Light Blonde to Very Light Blonde. I like to have a blend of shades.
Here is a link to their shades/website: ion Color Brilliance™ - Permanent Hair Color ShadesThey have some really pretty shades, but the developer is what has impressed me most. The website gives all kinds of instruction.