Hey bro. I've got a super easy way of doing white. Celestra Grey... Ceramite White. boom you are done. I hand painted the chevrons on my bikes doing that, white bits on my hornet rear wings, checkers, eagles, crux terminatus on my Dark Angels, etc.
I dont water it down or anything. Seriously Celestra Grey is a MUST have. You can even use Celestra Grey as a stand in for white. It so easy to use I do that a lot...
I also use it as the base to do my faces and what not. I paint the face all Celestra Grey and then do Kislev Flesh over the face for my Craftworlders, or Rakarth Flesh for my Dark Eldar.
Here's a few sample of my Celestra Grey to white...
This last one is just pure Celestra Grey with no white highlights. But you can see how close it is to white.
Anyway bud hope that helps! Feel free to ask as many questions as you may have. I know white is tough. Good luck!