I Had a similar situation to this the other day, i personally think its one of the hardeset tasks to go from a white blonde to only a few shades darker. i use wella so thought id let you in on what i did
first i weaved out some of her own hair in conditioner. then applyed 9/17 as the background however it just seemed to grab the tone. my mistake really should have known the base 9 wouldnt have grabbed! it was still as light but had gone a almost pink mink ish colour. (really nice colour lol just not what i wanted
) so we then had to again weave out the hair in conditioner and we appyled 99/0 over the hair. this time it looked lots better.
i think it does depend on your clients hair tho with white blonde as i guess you dont no how it will take the tint. some peoples hair ive done it has taken really well and some peoples just chucks it of lol, good luck with yours let us no how you get on xx