what is the neck wall thickness on a 300 win mag. And whats a good thickness to turn them down to.

Don't know the thickness but except for 1 target rifle with a custom chamber, I don't have to neck turn to specific dims. When, for various reasons, I do neck turn, I try to merely clean up the neck. That is to say, I set the neck turner to where it cleans around 75-80% of the neck.

Remington or winchester are about .016/.017 thick
I have never experienced much of a variation in thickness, the necks are pretty short and when you resize be crefull not to set the shoulder back .
Clean the inside and out side and make sure you are not measuring a burr .
If you do trim follow beeman's method.
If you do turn and do set the shoulder back too much expect splits. and letem fly
Dave