Hi there 400bullelk,
Quick question is the 400 actual or is it the dream [img]/ubbthreads/images/graemlins/wink.gif[/img]
There are a lot of fellas using Retumbo, may be a good start. Link
You have a good start with the rifle, reloading equipment and Chrono. You'll be enjoying things. The brake will make life good. I do all my 338 RUM shooting prone and its a piece of cake. Ya gotta love those good brakes.
I'd start with these basic steps which you will repeat many times.
Resise the new brass.
Trim to length
Chamfer case mouths.
Worry about neck turning much later - if ever (you'll get arguments here [img]/ubbthreads/images/graemlins/wink.gif[/img]
Pick a primer (Magnum of some manufacture)
Pick a load. I'd consider the one in the link, above.
Before loading consider ruining at least one case. It'll be worth it.
Take an odd ball unprimed case, if you sort by weight and a dermel tool with the cutter wheel. Make two cuts the length of the neck. This will give you a case that you may start a bullet into then chamber. If the neck tension is just right, when chambered with the bullet the bullet will be seated just touching the lead. This gives a good reference for carefully setting your seating die at a starting point.
As soon as you get a load that you like its time to start stretching the distance at gongs, rocks and random things.
The final step is a tag and a 400 bull.