resistant gray

hi,
I have very resistant gray hair, only about 10%.
I dyed with a high quality color(LK) recommended by
former colorist now beauty store owner. It is a level
7/0 called medium blond, permanent creme color.
I used a 6% 20 volume buffered developer.
I am very happy with the color, and it has given
my thinning hair more volume, but it hasn`t grabbed
the gray. He said to cut the developer with water in
half to 10 next time, because my hair is very fine
and this is why it isn`t taking on the gray.
Does anyone else have this problem and what do
you do? I read on this site that a colorist recommended
putting 20% peroxide on gray resistant hair for 15 minutes
before coloring. There is no way I can do this, my hair
is very fine, wavy, and thinning, it will fry. It always
reacts badly to any time of overprocessing.
Will cutting the developer help? Another site recommended that the color I used didn`t have neutral
pigments and that is why it didn`t take. I have used
one other permanent/water waves to cover it,
a light brown, it came out 2 shades darker than my
natural hair, was not a nice color and didn`t cover
the gray either.
My natural color is dark blonde to brown now with
lots of red/gold tones, mostly red. It was strawberry
blonde as a child. I don`t know what level it is
naturally, but in the past when I would dye it
with dark blonde dyes it wouldn`t change at all,
now I think it has gotten darker, due not going
in sun, aging etc.
I was thinking of mixing in an ash tone next time
since this one comes out more reddish on my hair,
would this help with the gray?
any advice appreciated.