The real trick to building a Weatherby that shoots is minimum headspace (.001 or less)
At the risk of being told I am wrong, I will offer the following advice.
If you have such a well cut chamber, then headspace on the belt. If your chamber is cut sloppy then headspace on the shoulder to compensate for the excess.
With a minimum headspace chamber the soft brass will like to grow right in front of the belt and your resizing die will not do the job of getting it back down to where it will chamber again. At this time you have three choices. 1 get new brass. 2. Order the belt squisher die from the guy in Florida. 3. grab up your full lenght die and slap the base down on a belt sander until some is gone.
If you choose #3 then you will need to run a piece of fired brass through it and carefully adjust the die to just meet the belt. Once you know where that is then set the lock ring and you are good to go.