Molly,
I think the point is that you want the liner to JUST not interfere with the threading of the breach plug. Get it as close as possible, but make sure you can still remove the breech plug without removing the liner. With most calibers, except for those that are very small, this will allow you to get powder in through the vent in the case you happen to dry-ball the gun. And this also give you a good ignition point. Contrary to what some people believe, the powder in a gun burns... It does not explode. Yes, it burns very rapidly, but that is not an explosion. Some also believe that it is a fusing effect of the powder in the pan that sets off the main charge... It is not. It is the flash of heat from the pan powder that ignites the main charge.
Matt
