That sounds reasonable. I'll try post the results of my tests on some old monopose grots.
I've thought about the official space wolves paint scheme?
https://www.youtube.com/watch?v=oKdzNh1JGE4
But it hasn't got that grey to it. I'm not using Dark Reaper as I have the Fang Spray. I'm also missing Codex Grey and Rhinox annoyingly.
So how about:
Black, Eshin grey, Eshin mix with The Fang, The Fang, Then the Fang mix with Russ Grey, Russ Grey, then mix with Administatum.
I already know I'd do the leathers as Rhinox then Gorthor (or Steel Legion Drab) then Baneblade brown (since that's got a grey to it).