Doublet, looks like what you've made yourself is pretty much a copy of Stoney Point's Chamber All guide. See stoneypoint.com I use one to seat my bullets out accordingly from .010 to 040 off the lands. As far as brass. a once fired pice should give you a good idea of the overall chamber sizing accounting for about .002 or so spring back in size (reduction). Sinclair International (www. sinclairintl.com) also sells a chamber length guide much can be used to determine the neck length or overall chamber length. I found this exceeded the SAAMI spec max on my .308 but turns out it has little overall effect on accurancy as far as I can determine to date. I've sized 30-06 brass down to .308 win for the greater neck length but as of yet it hasn't made any benefit in group size. This was a pain in the butt, fair amount of time to size down, anneal, do some neck turing to remove excess brass, etc.
Regards,
Travis