Once I decide this question, then I can order my reamer... I've (we actually) decided to go with a .358 WSSM.

So, for a hunting rifle, I'm obviously not going to use a tight chamber such as 0.002" of neck clearance...

But what is a good place to be? I've seen reamer prints everywhere from 0.002" to 0.006" and how much will it matter on a 300 yard gun?

I've already ordered "standard" dies from Redding, whose reamer drawing says 0.390". But it seems that turning the necks to ~0.012" to 0.014" would be required...

I'd rather not turn the case necks except to get rid of the "donut". I'm getting a consistent 0.016" to 0.017" after necking up the brass, except for the donut which is right at 0.020" to 0.021".

So after turning the donut back to 0.017" that all adds up to 0.392".

My though is to go with 0.395" or 0.396" for the neck diameter on the reamer. What are your thoughts?

The .399 that the HDH (WSSM) used requires not neck turning and is what Dave Manson recommended. And I doubt you will have that doughnut since you are necking up instead of down. The doughnut does not matter anyway except on cases prepped for extreme accuracy. The only case I remove the doughnuts on are my 1000 yard .50 BMG match cases.
On a fairly short range rifle you sure would be better off with a bit much than too little on the chamber clearance since on a hunting rifle it is not improbable that lint or some other crud could find its way into the chamber and could keep a round from fully seating.
Personally I'd go with what Dave recommends since he has been doing wildcat reamers for a whole lot longer than most of us combined. With a .017 neck that only leaves .006 inch clearance on a .399 chamber and that is not all that much, being only .003 per side. Much less than that and you could run into chambering problems if dirty or when the normal case neck increases in thickness due to repeated firings.

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Well, it's a moot point now - neck turning is inevitable.

We got our dies (Redding 358/25 WSSM, P/N 80739), and I took some measurements (we happen to have a nice set of gauges here at work) and neck turning will be required for the dies to work properly.

Appropriate neck thickness for these dies is going to be 0.015" or less. Dave Manson has already made the change on my order, and I will be getting a 0.390" neck dimension in my chamber.