To measure case head expansion, you need a micrometer that reads to .0001" (1 ten-thousands of an inch). A normal .001" micrometer isn't accurate enough for this type of measuring.
You will also need to 'scarifice' around 5 cases per batch if they are rimless or belted. Rebated rims do not need this done.
To accomplish accurate readings, you need to index (draw a line across) the bottom of the cases with a felt tipped marker, and then at opposite ends as indicated by the indexing, you need to file off about .003" of the rim, very carefully so as not to touch the case body, this is so the jaws of the micrometer fit at the very rear of the case body just forward of the extraction cannelure, this is where the measurement is taken. If you have 'blade' type micrometers, you won't need to do this.
The procedure is to take your reading as described above, a few measurements at the same place is the most accurate, you can take a measurement with the case one way and then rotate it 180 degrees and measure again. Take note of your measurement. This measurement is to be taken BEFORE firing the case.
After firing the case, take another measurement at EXACTLY the SAME place on the case and record that measurement.
The difference between the 2 is the case head expansion; .5312" before firing and .5315", or .0003" larger, after firing.
I do this with 5 cases from the same batch for each load increment.
You should discard these cases after 3 firings, because work hardening can set in and give you erroneous readings. Always start with once fired (starting loads) brass, because the first deflection is already present.
It is a general rule that .0003"-.0005" is around 50,000cup, but I prefer to work off .00025" as maximum expansion allowed.
Once this is reached, I then reduce my loads so that no expansion, or only .0001" is detectable.
It must be noted, however, if ANY one case, from the same batch, shows expansion above .0005", the load is TOO HOT and you will need to reduce the charge back to the load that showed no expansion or only .0001" for safety. A change in ambient temp may cause the previous HOT load to go way over and ruin your day and firearm!
Hope this helps, if not, I suggest you buy the latest Speer Manual #13, which describes the same procedure.