Okay, lets start off explaining that "SAAMI" meaning Sporting Arms and Ammunition Manufacturers' Institute, Inc. Basically an institution set up in the USA in an attempt to provide a standardised playing field for the dimensions surrounding firearms measurements and testing standards, manufacturing standards etc.
They are guidelines so what happens in reality may not always be the case but basically they should be close.
http://en.wikipedia.org/wiki/Sporting_A ... _Institute There is also the European version. "C.I.P." Commission Internationale Permanente and they do try and get on together but not always the case.
http://en.wikipedia.org/wiki/Commission ... Portatives So basically if you have a USA manufacturer / calibre it will be specified in Inches and European will be Metric so be careful with the conversions.
These are standards that most firearm / ammunition manufacturers follow given their own quality control standards. A Custom built firearm does not necessarily conform to these standards but should be close. There are many custom chambered rifles with wildcat calibres and chambers that do not have any specific standard yet alone a SAAMI specification.
So onto Case Triming. For general standard calibres there will be a specification. Lets take the .243W as mentioned.
The SAAMI specification.
http://www.google.com.au/url?sa=t&rct=j ... Gc&cad=rja You will notice two drawings. One is the Cartridge and the next the Chamber Specification.
The maximum cartridge case suggested length allowable is 2.045" minus tollerence of 0.020" but in the next diagram you will see the Chamber Specification of 2.055" for suggested maximum length and if you approach this length you are looking for trouble and may not even be able to chamber that cartridge case. So we have two workable dimensions, 2.045" being maximum and in general most suggestion and tools are made to trim that dimension by 0.010" (2.035").
"on_one_wheel" In my opinion you are trimming too much by 0.015" from the suggested trim length and that is a suggestion only. Some trim less depending on the specific cartridge and it's growth rate after each firing. The shorter you trim a case neck the more likely you are to cause a carbon ring in the neck area of your chamber prior to the actual bore beginning.
You are correct on the tool meantioned to measure the chamber neck length, overall cartridge chamber length. As long as there isn't a carbon ring in there which will give you a false reading. Another method is to take a mould of the chamber and partly into the bore using a product like "Cerrosafe" Chamber Casting Alloy, easily melted and reusable.
This is not a proceedure the normal reloader would be doing and more likely with those precision reloaders with custom chambers trying to keep cartridge case length to a maximum and precise length. Mostly in the view of maintaining very consistant neck tension as case neck length variation does affect neck tension.
Most people starting off with reloading that trim cases to length will use something like the Lee Case Trimmer Tools. Cheap, easy to use, fairly quick but these have a set trim length, generally 0.010" less than suggested factory maximum length and not available in some chamberings, especially some specific custom chamberings that do not have a standard.
You will need a "Case Length Gauge" for each different calibre. This also applies to some other fancy Case Trimmers that are "Calibre Specific" so you will need one for each calibre case you want to trim. If you have a lot of different calibres this can become very expensive.
The Lee system. 22-250 Calibre shown.
As mentioned by "Ironsides" a lot better system and very adjustable is the "Wilson Case Trimmer". Can be adjusted to suit your own desired trim length but not only that can be used to "skim trim" a cartridge case head to square the head a tiny fraction better than it may be. For those long range precision reloaders it's also used to trim "Bullet Meplat's" if you are doing Bullet Pointing and desire that feature.
The setup for use trimming a Bullet Meplat prior to Bullet Pointing.
For those that may be interested the process for Bullet Pointing.
Normal meplat variations of Berger VLD Bullets.
Normal meplat, trimmed meplat and then a pointed bullet.
The Whidden Pointing Die in use, then the resultant pointed bullet.
I have probably missed some points to do with Cartridge Case Length Trimming and there are many products available that will achieve the objective. Just trying to cover a few.
Sorry, but it's been a long post. Hope it helps some reloaders.