Take a fired case, and an unsized bullet that slips into the case. Place it in the case with a few grease grooves exposed. Chamber the dummy round and close the breech. Open the breech and carefully extract the round and measure without moving the bullet. That is a good starting point for seating depth. My Shiloh only lets me seat the Lyman Postell (457132) to 1/2 the first driving band... perfect.
As for neck tension, I use some pretty good tension, as a result my case necks is worked pretty hard, and I have to anneal every 3 firings. Maybe about .003" tension (interference) or so. Others I shoot with use none. You can literally pull their bullets with your fingers. They think I'm insane. Conversly, I think that is ludicrous. I'm also not a big fan of exposing grease grooves to gain powder capacity. Dirt and grit on lube is not the key to long and healthy barrel life.
I'm assuming you are using black powder or Pyrodex. Most black powder and Pyrodex need some compression to work properly. Swiss BP does not. Measure how much of the bullet shank is seated in the case. Make sure there is NO airspace. That wrecks things - puts a ring in the chamber. Calculate your compression with the difference of the bullet shank in the case and the powder column height from the mouth of the case (uniform length is critcal). Example. My .45-90 uses the Postell bullet - .650" seated into the case trimmed to 2.4" on the nose. The top of the powder column in the perferred load is .580" (about 51.5 grains by weight/83 grains BP eqivallent by volume) from the case mouth with a .030" veggie gasket wad on top. .650" minus .580" equals .070" compression. Typically, that's about right for Pyrodex. Goex likes over .100" and Swiss almost none. Don't forget some kind of wad over the powder.
BP and Pyrodex have to be dropped through a copper, brass, or aluminum tube at least 24" to work properly. This allows the powder to settle and compact better. The best thing to do is find as many Mike Venturino, and Paul Matthews books and articles and read them.