Blonde over white/grey hair

[hennessy]

i am looking for a nice creamy pale blonde colour level 9ish for a client with 100% white hair. she doesnt want 'yellow' as she calls it so i generally try for sandy shades as im terrfiied to try any gold! she likes it as light as possible but with good coverage on the white. ive tried loads of different tint combinations but ive never really nailed it. last 2 were affinage infinity 9.035 but it had a pinky hue and then tried igora 9.40 but it was too dense/dark. just want some recommendations please. thanks.