Grey covering question

Hi fellow stylist ! Quick question on grey coverage, I have not done a lot of gray coverage formulation myself. I have a client who is very pepper, a few hairs here and there of white/silver. She wants to go back to Brown. I plan on using something in the N shades. I’ve had some hit or miss luck with those as far as coverage and deposit. (recently had a grey retouch go Green) all this to say what volume developer should I be using? I typically use Pravana or matrix but I’m also open to suggestions! Any tips are welcome and your suggestion for volume would be greatly appreciated!!