I am forming 300 savage brass from 308 cases and need to know what the neck thickness should be.
??

Neck thickness will be totally dependant on the dimensions of the chamber in the gun for which you are reloading. You would need to determine the diameter of the neck in that chamber. The Savage is spec'd .004" smaller than 308.

If you have a fired case from the Savage, measure the neck diameter. That should be close enough without making a chamber cast. From whatever that figure is, subtract the diameter of the bullet, .308" plus .004" (.312"). Divide the resulting number by 2 and that should be your maximum neck wall thickness. If the fired case measured .340", subtracting the .312" would be .028" so those walls should be .014" or less.

Bob338 said it well.

It is highly unlikely that the thinkness of the neck walls of reformed .308 will be too thick for your .300 Savage chamber; in fact, while they may be a bit thicker than factory .300, they will likely be just a little better fit.

Another practical test is to load a dummy round in the reformed and trimmed case and see if there is any resistance when the action is closed on the chambered cartridge. If your gun is the Savage 99, which has no camming power to push an oversized cartridge into the chamber, you will quickly know if any portion of your dummy round is too large. If there is chambering resistance, it could be that headspace is too long, your die is not small enough at the shoulder, or your necks are too thick. Treat the cartridge shoulder and bullet with candle smoke or some other staining media to determine where the bind is and adjust accordingly.

Steve: I think the 300 Savage link is broken on that page.

Wstrnhuntr: It's my understanding that a bullet should be able to drop into a fired case.

One method I used on 7TCU was to drop a bullet into the fired cases - any case that was too tight in the neck required neck reaming.

Rick.