FWIW, I recommend starting with a light gray and working up (think Celestra Grey - Ulthuan Grey - White Scar). It's rather easier than trying to use a shade
Alternatively, starting from white spray do use a shade - usually (thinned) Nuln with potentially some other shade mixed in - then a heavy drybrush of Ulthuan Grey, highlight with White Scar. Ulthuan Grey is your best ally in creating good-looking whites
, how are those dryads coming along?
Ask not the Eldar a question, for they will give you three answers; all of which are true and horrifying to know.