This is an example of a cartridge headspace gauge (Midway calls it a case length / headspace gauge)
http://www.midwayusa.com/product/880...308-winchester
I see they're out of stock. LE Wilson may have them, and they sell directly, I believe (not just to distributors). Dillon may have them in stock, and their pattern is similar to the Wilson. LE Wilson's web site has a pdf manual for use of their gauges. The instructions are generally applicable to any gauge of this pattern, regardless of who makes it.
These gauges have a hi/lo step on the bottom (for cartridge headspace) and a hi/lo step on the top (for case length). Both are important for safe reloading. Only one can be measured with calipers, and it isn't cartridge headspace.
There are far more complex tools for measuring cartridge headspace, but the Wilson pattern is a good place to start.
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based on rookie observations with my calipers
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Well, the problem is that cartridge headspace is the dimension from the head of the case to an established "datum" midway up the shoulder. It isn't case length, which is measured to the mouth. The difference between minimum and maximum is only 0.006" or so, impossible to determine by visual examination, or by estimating the position of one jaw of a caliper held to one side.
Clymer is one manufacturer of a true headspace gauge set, used by a gunsmith to finish ream a chamber when fitting a new barrel. It will be of absolutely no use in setting up a resizing die, as it is measuring the chamber, not the case.
You can download Clymer's catalog if you want to read up on chamber headspace gauges -
http://clymertool.com/catalogue/index.html
So, if you are ordering a headspace gauge
set, you may be ordering the wong tools. The Wilson gauge is one per caliber, not a set. And they cost $35 or so. The chamber headspace gauges are going to be $120 for a full set of 3 (GO, NOGO and FIELD).